A feed reader reads RSS or other feeds from web sites and turns them into usable content.
A feed reader is a computer program that can accept RSS or Atom feeds from a web site and convert those into links, images, and content that people can interact with.
There are many different types of feed readers. The ones that internet users are most familiar with are the ones they use regularly. Adding RSS feed subscriptions to My Yahoo! or My Google, or Live Bookmarks are what most people see.
However, many web browsers and many email programs have built-in feed readers. You can add your feed to Firefox or Thunderbird out of the box, or to Outlook or Google Chrome with a plugin.
The other important way that a feed reader can be used is to dynamically update a web site. An RSS feed from one web site could contain information that is important to visitors of another web site. If the site with the content provides a feed of that information, then the other site can use a feed reader to update their site dynamically with that new content. This can be useful for emergency services, event schedules, or general announcements.