A microbrewery, or craft brewery, produces a limited amount of beer. The maximum amount of beer a brewery can produce and still be classed as a 'microbrewery' varies by region and by authority; in the US, it is  a year.
Microbreweries, which make a small amount of beer, are defined differently depending on where you are. In the United States, for example, they can only produce a certain amount of beer per year before they are no longer considered microbreweries.