Microtonal music is any music that uses a tuning other than standard twelve tone equal temperament.
Twelve tone equal temperament is the basic piano tuning that we are used to. It really works quite well, which partially explains the fact that it is the only tuning many people are aware of.
I am a restless individual, however, and am not content with the the so called norm. A lot of microtonal music is based on various historical or ethnic tunings, but I am especially intrigued by certain experimental tunings that are hardly ever used. I am currently writing a symphony in twenty-six note equal temperament, an obscure tuning with strange properties. I hope to have some links to my music soon.
See also What is Xenharmonic Music?