An aggregator is software that displays the feeds to which a user has subscribed. People use aggregators primarily to have one central place that shows them what's new on sites or topics they care about.
A blog aggregator allows a users to click through the latest postings from the blogs she has subscribed to. But aggregators usually allow users to subscribe not just to blogs but to any source that outputs a feed of its latest information. That includes many tagging sites. By subscribing to a tag, users can see all the objects other users have tagged that way.