Tim Converse, the web spam fighter at Yahoo Search, wrote a very interesting blog entry explaining aggregation spam. In short, aggregation spam is a form of content spam where you scour the web for matches on a specific keyword phrase, then compile a page of content with snippets and chunks of content found containing that keyword phrase and related keywords around it.
Tim offers up this extreme analogy;
Imagine that you get home one night to find a stranger leaving your house with a sack containing your TV, cell phone, jewelry. You might misunderstand, until we explain that he's actually an aggregator - he's just aggregating your belongings.
Tim explains that it is hard for the search engines to draw a fine line in the sand as to what is defined as high-quality aggregation that should be included in the search engines versus those that should not be included. But one thing he personally believes is that the "the bar for inclusion ought to be pretty high."
Read Tim's personal thoughts on aggregation and search at his blog.