One of the more recent Internet marketing trends lies with creating microsites which are small Web sites usually numbering under fifteen pages dedicated to one specific niche topic. Often times, sites will set up microsites to rank for a specific term or set of terms with links back to the parent domain. While there are several arguments for and against the creation of microsites, whether or not you'll benefit from a microsite depends on the implementation and what you want to get out of the site.
Let's say you own a moderately successful site that sells power tools and you thought it would be a great idea to add a section about what each of the tools on your site could be used for. There are two ways to go about this. The first would be to create a new section for do-it-yourself (DIY) projects. In order to support the main focus, power tools, link the DIY section back to the products and vice versa to flow traffic through the DIY section. The only problem with this is that while a designated section for DIY projects is great, it can dilute the main theme of the site which is power tools. In addition, there wouldn't be too many inbound links to the DIY section since users are coming in through the power tools side. The only way to find the DIY section would be to navigate through the site.
The other option would be to create a microsite specifically for DIY projects. Ideally, you would want to find an aged domain that has had a real site on it for a while that also has some of the keywords you want the microsite to rank for. This opens up a whole new arena because now the DIY site can be optimized for DIY terms (building a deck, floor installation, etc.) while the main site can be optimized for all the power tools terms. If you're starting fresh with a new domain, the search engines will allow you to link between the two sites as testimonial-grade links to get your inbound link count up. With links pointing from specific products on the DIY site to product pages on the power tools site, the inbound link count increases as well. Keep in mind however that microsites don't give as much PageRank value as would have been given with another section on the main site.
When deciding whether or not to create a microsite, think about the themes on your main site. If your site is overly generic or is disjunctive by nature (power tools and DIY projects), you may want to consider creating a microsite. The generic site may have trouble getting inbound links simply because it covers too many topics without enough depth. On the other hand, the microsite won't be able to offer as much of an SEO benefit as far as links, but starting with something is always better than starting with nothing. In the long run, microsites can generate more inbound links than creating another section on your main Web site.