An aggregator, or feed aggregator, is the same idea as an RSS feed reader. An aggregator is a piece of software or an account on the web that is used to collect or subscribe to RSS feeds. Bloglines.com is a free online RSS feed aggregator--one place to collect the RSS feeds to which you wish to subscribe.
iTunes is even an RSS feed aggregator for podcasts. You can subscribe to podcast series in iTunes. Each time a new podcast is created in a podcast series to which you have subscribed, iTunes automatically downloads it to your computer for you.
See also: RSS
Wikipedia Link: Aggregator
The above Wikipedia article includes a list of links to several aggregators for RSS feeds, media feeds, and assorted feeds.
Dictionary.com Link: Aggregator (about the 3rd definition)

