Micro sites, Satellite websites or Niche sites, or whatever you want to call them. They are small websites that provide support to your main (selling) website.

You can use them for several reasons. First, they provide a way to control the number of backlinks to your main website. The anchor text, the target url and the location of the link can all be determined by yourself, which gives you a powerfull tool to push your main website higher in the rankings. A lot of backlinks will point to your main page or home page but with Micro sites you can also point to other pages, and thus creating deeplinks.

A good practice is to have several Micro sites, each performing well in it's Niche which should be related to your main website. Each Micro site can also be cross linked to support the ranking of each Niche site.

The other reason to use Niche websites is to perform pre-sales, that you would normally not get with your main website. Since the Niche site is very focussed you can pre sell your product and then direct them to your main website.

A Satellite website should have 5-10 pages, but preferably more. Try to get some relevant backlinks and fresh content for them, so they will rank well on their subject. The better your Niche website ranks, they more it will support your main website.

Alan Bleiweiss has written a very good article on Multiple Niche sites