I have been asked to explain what is meant by "news", "newsgroups" and "Usenet", and the difference between a newsgroup and a mailing list, and I'm happy to oblige.

NEWSGROUPS

What are Newsgroups?
Newsgroups are round-table discussion forums.  They are just about the oldest part of the Internet, and originate from the days when there were many different on-line networks - Compuserve and CIX were two commercial examples; Fidonet was the major hobbyist example.  Academic Institutions had their own networks, one of which was (and still is) Usenet - the Users' Network. In 1993, Usenet started to be distributed using the then fledgling Internet, and became open to all, an event still mourned by a few diehard Usenetters.  This history means that newsgroups are different from almost anything else available through the Internet - for better or worse, depending on your point of view.
The term "newsgroups" is a misnomer.  Newsgroups have nothing to do with News as generally understood. They are discussion groups. Every newsgroup has a topic, as defined in its charter, and usually obvious from its name. Most newsgroups are strictly text only, but there is a set of newsgroups, known as binaries, whose purpose is the distribution of pictures or, less commonly, sounds, movies or software. As an example I'll use the group known as uk.railway . The topic of this group is railways in the UK. Somebody will make a post, known as an "article", to the group, which could be anything as long as it's on topic.  The post won't be to anybody specific, but to the group as a whole.  Hopefully, somebody reading the article will have something to contribute, make a reply (again, to the group as a whole, not an individual), and, from that original article, a thread will develop.  The thread may  veer far from the original question (but still on topic), but due to the way news is transmitted, by a system known as NNTP, keeping track of threads is made easy, especially if you use specialist newsreading software. So far, this sounds much like the mailing lists run by Yahoo and others (including me!), and which you may subscribe to.  The essential difference is that there is no central focal point; nobody running the strings; no single archive.  There is (usually) no moderator, and the group is propagated through many news servers throughout the world. Anybody can subscribe.  There is no list of members.  You can come and go as you please, and do what you like, provided you don't upset too many others in the group.  If you do, they will let you know in no uncertain terms, and if you persist, they may well send an abuse report to your ISP which, in the most severe case, may lose you your account. Most (but not all) ISPs run news servers as a service to their subscribers.  There are also many thousands of other news servers, both public (free and commercial) and private. I subscribes to a commercial news server, Easynews, as well as making use of a free German server, news.cis.dfn.de. Within Spain, the only major ISP that doesn't run a news server is (as far as I know) Eresmas.  Terra /Teleline / Telefonica and Wanadoo both run reasonable news servers. 

There are, probably, something like 60.000 newsgroups.  No server carries them all, although Easynews carries 40.000 and Wanadoo over 30.000.  However, even Terra's 20.000 is enough to satisfy all but the most esoteric of tastes.  If you want to get a taste of Usenet without resorting to a news client, there are various web based news services and archives. Probably the best, certainly the best known, are the archives at Google Usenet Archives, an offshoot of the Google web search engines
 (http://groups.google.com).
If you want to discover more about the world of Usenet, there is only one place to go:
http://www.usenet.org

How do you get started?
The first thing you will need is a newsreading client.  Although Outlook Express is often used as a newsreader, especially by beginners, you may find it worth downloading specialist software.  By far the most popular is  Free Agent (because it's free!), which you can download from http://www.forteinc.com
Having opened your news client, you will need to download your ISP (or other server's) newsgroups list.  This will take some time, but only needs to be done once (except for periodic updates). If you download and run Free Agent, it will take you through this step after you have entered your NNTP and SMTP servers. Free Agent needs details of an SMTP server to enable you to make a personal e-mail reply to a Usenet post. 
However, if you've ignored my advice, and are using Outlook Express do this.  Go into Tools > Accounts > Add > News.  Enter the name that you wish to use in your newsgroups posts.  Click "next" and enter your e-mail address. Click next and enter the name of your NNTP server.  This will be something like news.myisp.es.  Most news servers  do not require you to log in, but if yours does, enter your name and password after checking the box. Once you have checked the "finish" box, you will be invited to download a list of newsgroups.  Make sure you are connected to the Internet, click "yes", and make yourself a well deserved cup of tea while the download takes place (usually between 10 and 20 minutes).  Now you will be invited to subscribe to newsgroups, by typing in a piece of search text.  This is a crude way of finding the groups you wish to subscribe to.
You will now find that your news service has been added as a new folder.  Most actions to do with news are done by right clicking. You can bring your list of newsgroups up to date, subscribe and unsubscribe from groups, and download new articles (which OE quirkily calls "synchronising").  Unless you have a full time connection, it's worth downloading both headers and articles in one go - being plain text, articles don't take long to download. Select a newsgroup.  The article headers will be in the top frame, their contents below. For the time being, content yourself with reading others' contributions.
If you are using Free Agent, the steps are very similar to those above, and most of the default Free Agent settings will be just fine, but as with Outlook Express, the caveat about working offline and downloading headers and bodies in one go applies.  It also may well pay to limit the number of headers and bodies retrieved on one go.  You do this in Groups - Default Properties - Retrieve.

How do Newsgroups get distributed?
When somebody uploads a newsgroup article to their news server, it is then distributed to all the other news servers that carry that group all over the world. Subscribers will download the article from their chosen server.  All servers will only retain the article for a fixed period of time, typically a week.  After that, it will be deleted, known as "expiring".  When you subscribe to a group, and choose to download all articles, you will find that nothing is older than a week or, just possibly, two weeks.  In the case of binary groups, the expiry time can be as little as 24 hours.  Therefore it pays to collect your news regularly.  Unlike e-mail, you don't have a personal "mail box"; you download from a central pool.  If the article has expired, you're too late.

What are binary groups?
Binary groups are groups that carry articles with attachments, such as pictures, movies, sounds or software. Most of these groups are totally innocuous. However, be aware that others carry pornography or pirated software and music. The bandwidth (size) of a binary article is typically many hundreds of times larger than that of a mainstream text article.  This means that many servers limit the number of binary groups they carry, and give them short expiry times. If you wish to subscribe to binary groups, be aware of the implications.  Firstly, downloads will take a considerable time with a normal analogue modem. Secondly, it's very easy to post a virus carrying executable attachment to a binary article, so take the strongest anti-virus measures at your disposal. Finally, make sure your server carries the group you want (Wanadoo seems to carry everything!). Specialist software is available which makes the use of binary groups easier.  Outlook Express is hopeless, and Free Agent merely acceptable, if you are downloading many megabytes of software or images, many of which will have been posted in several parts.
 
Posting to Newsgroups.
OK, so you've been reading a group for a couple of weeks, and think you have the "feel" of it.  Somebody has made a post and you find you just must reply. You will be fine if you keep to these rules, which I have put in what I think is order of importance.
Post using plain text 
Don't post upside-down.  Replies go AFTER the text they're referring to 
Don't over or under quote.  Quote just enough to make your context clear 
Try to stay on topic (some groups are stricter on this than others) 
Remember that your post becomes public property.  You have no control over who reads it or replies. 
For this reason, if something starts to become private between individuals, take it to e-mail.
Most newsgroups have at least one self-appointed guardian who will be happy to pounce if the newcomer steps out of line. Ignore them!  You will find everybody else does.  However, if you are unfortunate to suffer howls of rage (flames) from many subscribers, then try to find out what you are doing wrong. 

MAILING LISTS

These are similar to newsgroups in that they are round-table discussions of a particular topic.  A great many mailing lists are private - these we will pass by, but many others are public in the sense that anybody can join.  Unlike newsgroups, they have a single, central focal point through which all messages will pass and be processed.  Distribution is by e-mail, and therefore the mailing list (or group, the terms are interchangeable), is usually referred to by its e-mail address, for example computerchat@billnot.com , a mailing list address referred to in every one of these columns.  A mailing list will have an owner, and it is the list owner who has absolute authority over what can or can't be done or said in the list.  Owners come in all shapes and sizes, some very tolerant, some very intolerant, sole lazy, some hard-working and excellent. 
The biggest single provider of mailing lists is Yahoo, accessed through
 http://groups.yahoo.com
There are literally hundreds of thousands of Yahoo groups, and a search facility is provided to find those that interest you.  However, anybody can start a Yahoo group (even you) and you will find the vast majority have little or no traffic.  The best groups have many members and at least a few posts every day.  The statistics can be found from the group web page before you decide to join. Most groups are available for reading and posting from a web page as well as by e-mail. It is a powerful and flexible system.

To join a Yahoo group, you will need a Yahoo i.d. and password.  Their website walks you through these steps.  You should be aware that Yahoo is a commercial operation, and it makes its money by showing adverts both on group web pages and in e-mails, and it makes no bones about collecting your personal details and using them - for spam if you like.  If you feel particularly strongly about this, it is better not to join a Yahoo group as you will almost certainly receive unsolicited e-mail.  But then the same can be said for Usenet and almost any public exposure of your e-mail address.