Note this was created in July, 2011. Passive Guy hasn’t checked to see if things have changed with the aggregators since then.
From Publish Your Own Ebooks:
In some cases instead of publishing directly to an ebook store you might choose to publish via an ebook “aggregator”.
What is an Ebook Aggregator?
An ebook aggregator deals with ebook authors directly and interfaces between them and ebook retailers such as Apple and Sony.
The ebook aggregator may offer other services besides distribution, for example ebook design and formatting services.
Link to the rest, including the footnotes, at Publish Your Own Ebooks