It's a bit of a misnomer these days. 10-20 years ago microbreweries were aptly labeled small breweries which were building a reputation for brewing more worldly and atypical beer styles such as various ales and lesser known lagers at a time when the big name lagers such as Budweiser and Miller were mostly all that was available. The beers microbreweries brewed became known as microbrew accordingly.
Many of those fledgling breweries have done quite well for themselves and have surpassed the legal production definition of what constitutes them being "micro" breweries though. In order to move away from a label based on output the term "craft beer" and "craft breweries" is becoming more common and arguably more accurate since it reflects their efforts to produce something a bit more niche and with a bit more care. This isn't always the case, there's certainly bad craft beer, but you get the idea.
