A microsite is a web site that has a small number of pages, usually no more than one or two dozen pages. Once the pages are created, they are usually not altered very often, if at all. To give you more specific ideas, here are some example microsites in our academic context.
A page about their group, a page about their group members, a page that lists their publications, and a page of their recent grant awards. Could also include blog posts by members as a form of news update.
A page about the club, a page that lists their offices, a page that lists when and where they meet. Utilizes blogs for event recaps or news.