If you are an active WordPress blogger and you have many different categories, it may be helpful to provide category-specific feeds so that your users can subscribe to the posts that interest them the most. For example, let’s say you blog heavily within five categories – Personal, BuddyPress, WordPress, Family, and Photos. You may have some readers who are really only interested in the WordPress and BuddyPress posts. This is how you can offer your readers the option to subscribe to one or more of your categories or tags:
http://www.example.com/?cat=42&feed=rss2
or
http://www.example.com/?tag=tagname&feed=rss2
or
http://example.com/category/categoryname/feed
Source: WordPress Codex – WordPress Feeds
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I wrote a little piece about this a while ago, looking at how to use all the available RSS feeds wordpress automatically generates for you. If readers here are interested it goes a bit further than this tip and it might be useful for readers here, so its at http://www.robparker.org.uk/2009/07/30/wordpress-rss-feeds/