RSS (Real Simple Syndication) is a format for delivering regularly changing web content you subscribe to directly to you through software (Reader) instead of visiting each site. It makes it really easy to stay on top of all your favorite sites by combining all the content in one place and delivering it right to you!
Feed Reader or News Aggregator are software that actually collect and display the data from RSS feeds for you. Some are web based, some you download right to your computer. Here are a few of our favorites:
Google Reader: http://www.google.com/reader/
Super easy if you already have a Google account
Great app for Mac users
Great for Windows users
Very friendly web interface