Product Focus
Lets say you sell televisions and want to take an advert out on one model of television and wish to promote it separately with its own domain name, this is when a microsite comes into play.
Microsites can be more than one page but the important thing is the majority of the links will take you to the main parent site to make a purchase for example.
Example You might have a microsite with editorial about Christmas using the editorial links to drive sales and traffic to the parent site which sells christmas decorations.