What are RSS feeds ?
RSS feeds allow users to aggregate frequently-changing content from various web sites (such as news headlines, blogs, discussion forums) in one place. You do not have to visit each web site and check for updates since your last visit: instead, new content comes to your RSS feed reader (such as Firefox, Internet Explorer 7, Thunderbird, Outlook 2007...)
Definition of a RSS feed in Wikipedia.
RSS feeds are a standardized web feed protocol. Users subscribe to RSS-enabled content in a similar manner as subscribing to newsgroups. RSS feeds are typically exposed by a web site (such as a web discussion forum, a blog, a news site) and most modern browsers are able to detect them and show a special RSS button in order to view and subscribe to RSS feeds.
Note that RSS provides read-only information. Unlike NNTP (the newsgroup protocol), RSS does not allow to post new messages. Instead, messages should be posted from the web site providing the RSS feed (typically a discussion forum or a blog).
Subscribing to RSS feed in Internet Explorer 7
Subscribing to RSS feeds in Windows
Internet Explorer 7 (and other browsers such as Firefox) has built-in support for RSS feeds. They typically show the RSS icon (see screenshot) when a RSS feed is associated with a web page. Clicking on the RSS icon allows to view and subscribe to the RSS feed. The RSS feed is then typically added as a special entry in the browser's bookmarks for quick access.
Checking for new items in a subscribed RSS feed (Internet Explorer 7)
Users of Outlook 2007 will see the RSS feed subscribed in Internet Explorer 7 as Outlook folders. Outlook regularily queries the RSS feed for new items and keeps a copy for offline access, providing a convenient way to view RSS feeds.
If Outlook 2007 does not show the RSS feeds subscribed in Internet Explorer 7, please check that the setting Sync RSS Feeds to the Common feed List is enabled in Outlook 2007 > Tools menu > Options > Advanced options. More details in this help page.
Subscribing to RSS feeds on Linux
Documentation on RSS clients for Linux can be found here.