How to Provide Category or Tag-Specific RSS Feeds For WordPress

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 that 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