What is RSS?
Depending on what site you look at RSS has a couple of different definitions. Some say RSS stands for Really Simple Syndication, others say Rich Site Summary, or is it RDF Site Summary. Whatever you call it simply means that you are pulling content from another website for display on your own site, in a standalone RSS aggregator or by other means.
RSS comes in a couple of flavors. Most common is a headline and description of what is new on the site you are pulling the RSS feed from. In most cases the feed will be in XML and look something like:
<rss version="0.91">
<channel>
<title>Developer</title>
<link>http://www.internetnews.com</link>
<description>Developer</description>
<language>en-us</language>
<copyright>Copyright 1996-2003 Jupitermedia Corporation</copyright>
<docs>http://backend.userland.com/rss</docs>
<managingEditor>feedback@jupitermedia.com</managingEditor>
<webMaster>webmaster@jupitermedia.com</webMaster>
<item>
<title>Business Objects's Plan Becomes 'Crystal' Clear</title>
<link>http://redir.internet.com/rss/wd-news/www.internetnews.com/dev-news/article.php/3296791</link>
<description>The business intelligence software giant outlines its tricky product roadmap for its purchase of Crystal Decisions.
</description>
</item>
<item>
<title>IBM Releases WebSphere 6 Sneak Peek</title>
<link>http://redir.internet.com/rss/wd-news/www.internetnews.com/dev-news/article.php/3296741</link>
<description>The application server code base is seen giving developers a head start on J2EE 1.4 applications.</description>
</item>
(etc....)
Why use RSS?
Many people like to be notified of news and updates from their favorite site without having to visit it directly. Others would like to include content from a site that is related to theirs without either violating the original copyright or taking people off of their site. While yet another reason is to be notified when their favorite information place has updated its content.
To allow subscribers to get your newsletter without being reported as a spammer. Even double opt-in email newsletters that require a multi-step process to subscribe get reported to SpamCop and other anti-spam organizations that result in their newsletters being blocked as "spammers" frequently because someone either forgot the subscribed or were tired of receiving the newsletter and didn't want to spend the effort to unsubscribe properly. (Note this happens frequently with AOL.)
In some cases entire newsletters maybe syndicated by RSS and placed on your site. Lockergnome one of the best known technical newsletters offers both a short version (new) and a longer full version of hate newsletter you can put directly on your web page. Examples of both are contained within this section of the Web Technologies stile.