RSS stands for Really Simple Syndication (it originally stood for RDF Site Summary -- see an RSS history) and it is a machine-readable format for delivering syndicated content. It really is not a single format but a collection of formats; the whole list includes RSS 0.90, 0.91 Netscape, 0.91 Userland, 0.92, 0.93, 0.94, 1.0 and 2.0, as well as the new Atom standard. There is a fair amount of history behind the different formats exist; the main consequence is that sometimes a reader will not be able to process some content, although the readers are moving towards accepting all content.

RSS is often used to read blogging (web logging) and news. In both cases a source (a feed) is identified by a URL. The feed contains a machine-readable description of the content available, a short description, and a URL for more complete information. A reader program may keep a list of feeds and also a list of items that have been read so it can only show new content.

The same way that most people do not read raw HTML, most people do not read raw RSS feeds. Instead, they, directly or indirectly, rely on some software to read the new feeds. The most popular approaches are:

Visit an web page that will do the work for you. The categories are a bit fuzzy but for the most part they can be described as either plain web readers, aggregators or search engines,
Use a specialized desktop reader tool to query the feeds, or
Use a browser or mail reader that recognizes RSS directly, like FirefoxAndThunderbird

Some good pages for introductory material on RSS are available at Google and others.

Publication Dynamics

TBD -- Authors, Readers, Bloggers, News Feeds, Search Engines, Cross-Links, etc. ?

Projects at Java.Net

There are a number of RSS-related projects at Java.Net. Some of them seem a good fit in WS and XML community -

P@: Adding a few comments about the projects you mention, in order to avoid an apple to oranges comparison!

rome - a set of Atom/RSS Java utilities to support most syndication formats. (P@: 5 active developers, tons of documentation, active traffic in users and dev mailing list, enters beta soon, handles all formats, many subprojects)
[general] j-rss-bayes -- (http://j-rss-bayes.dev.java.net/) bayesian filter (P@: this is not a project, it is just a 3 lines proposal from 18 months ago, with 2 email messages asking if this project is useful, nothing else. You should delete it from the community if his author agrees)
[general] jrss - (http://jrss.dev.java.net/) rss library (like rome) (P@: updated last march 2004, 1 developer, no traffic in mailing lists, no docs, seem to have a small ui and handle only rss 2.0 and its ancestors: don't compare that to Rome please;-)
[javadesktop] fetchrss -- (http://fetchrss.dev.java.net/) but never used it. (P@: it's Crazybob's RSS to email gateway, 1 developer (but very good), haven't seen any docs. Updated last time august 2004, stable, no more active development. Not sure which versions he handles.)
[javadesktop] planet-on-wire -- desktop RSS client
[javadesktop] rssclient -- but won't consume RSS 1.0?

Other Software Projects

UFP (python-based - Universal Feed Parser)

Desktop Readers

There are a number of desktop tools that are specialized for reading and/or producing RSS feeds and/or Blogs. Some popular tools are:

Mac OS X -- NetNewsWire, and its free lite companion.
Windows -- SharpReader.
Linux -- Straw.

Browsers and mail readers are favorite tools for information handling and they are beginning to include some RSS support. My current favorites are the Mozilla products:

Multi-platform -- FirefoxAndThunderbird, the latest browser and mail reader from Mozilla.org.

Older browsers can also support RSS reading either through a plug-in or just by using Javascript plus frames. For example:

Browser-based -- RocketInfo, NewsMonster.

Web Readers

Bloglines is a very popular web site that can be used to do blog reading...

Server-side -- Bloglines.

Aggregators

Aggregators are web sites whose main purpose is to aggregate popular RSS feeds into HTML content that can then be consumed through a browser. Some times the aggregator will personalize the aggregation, sometimes they do not.

Here are some in the area of the Java platform

Search Engines

Search Engines are web sites and/or services that search RSS feeds. There are some variations on the idea, here are some popular ones:

Technorati -- A real-time search engine that keeps track of what is going on in the blogosphere - the world of weblogs.
Feedster -- Search engine and aggregator.