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Basically you pass the feed url to this class and it returns an array containig the parsed feed information in a ready-to-use format. You can use this array to display the RSS content in any way you want.
The top results in this search has some decent summaries: What is RSS? [google.com]
Basically, RSS is an easy way to syndicate news or content from your site. This is a great way for people to keep up with what's new on your site. If you provide an RSS feed then people with aggregators can check that file for updates. Some people use it for newsletters because unlike e-mail, we don't have RSS spam. It's a PULL medium and a lot of people like that nowadays.
RSS is pretty cool, kinda like getting an email alert when any pages at your favorite sites are added or edited.
Opera 7.5 has a built in reader for rss feeds. If you don't have Opera, Yahoo's got a rss mod that you can use on your My Yahoo account. It also has the benefit of being pretty dummy proof as it took me a couple hours of messing around to get the Opera settings right. MyYahoo was grabbing feeds within a couple minutes.