Micronesia is a collection of hundreds of small islands in the Pacific. It's basically the area bounded (loosely) by Kiribati in the east, Wake to the north and Palau in the west.
Many of the islands are independent but some are territories (such as Guam). US interests such as Guam and Wake are administered by the US but they are not actually part of the US. Wake is (or at least was) restricted access and really only used by the US Army and US Airforce. I'm not sure if they're still there or not.