Creating categories in the aggregator

1. Go to administer » aggregator then click on the add category tab.
2. Add a title to the category, then a description.
3. If you wish to have a block of the last x items from that category, select the number of items in "Latest items block". To place the block on your sidebar, go to administer » blocks and look for the category you just created.

Now every time you add a feed, you can select a category which the items will automatically appear under. Alternatively, you can tag individual items in your aggregator to appear in a category.