Micro sites are small sites that are focused on a very narrow topic. They normally have the topic (or keyword) as part of the domain name. These have been used to try to get rankings on very competitive search terms that the existing website is not having much success in.   They rely on the search engines tendency to place some importance on the domain name thereby giving the website a ranking advantage for a keyword that is in the domain name. 
Like everything else in the world of SEO, when something is successful, that concept will be used and abused ad-nausea.   Any overuse and abuse has historically led Google to modify its search algorithms to reduce the effectiveness of whatever trick the SEO specialist was trying to use. Not surprisingly, Google has reduced the importance placed on domain name. It still has some relevance and if there is an opportunity to get a domain name on a new website with good keywords in it, then do so.
Over the last couple years, I have found it more advantageous to focus on the larger main website than developing micro-sites.   I believe that in most cases you are better off creating specific pages or sections of your site focused on a topic rather than building a bunch of smaller sites. The reasons for this:
·         Larger sites tend to rank better than smaller sites
·         Your main site usually has some authoritative inbound links- your are starting over with a micro site
·         The domain age is still an important factor – adding to an older domain name may work better than creating a brand new one that tends to be ignored for while
·         Adding and changing content on your site is very beneficial to rankings
·         The benefit from keywords in the domain name has diminished
·         It is harder to find domain names with good keywords
Micro sites can still work and there are some very good reasons to use them:
·         You want to place more emphasis on a specific product or service that you offer and feel that it is getting “lost” in your existing website
·         You are concerned about “Brand Confusion”
·         You want more focus on specific geographic areas
Whatever the motivation, if you do chose to create a micro site (which will probably grow into a larger site) you still need to consider the following:
·         Unique and engaging content is a must
·         Good inbound linking strategy
·         Avoid using the micro site for the purpose of creating links to your main site
·         Frequent changing and updating content
If you feel that someday your small site will grow into a much bigger site, then a micro site is a good strategy. If your intension is to create a 3 page site and never look at it again, then don’t bother