Microbreweries make less than 15,000 barrels per year, and craft breweries make less than 6,000,000 barrels per year.
Aside from this distinction, are there any differences in the styles of production by the two entities? If there's no difference in production technique, why make this distinction at all?
That is the difference. The distinction is purely one of size, and exists mainly for purposes of trade and legal organization. The gulf between the two wasn't always so large, but as the Boston Beer Company (Sam Adams) has grown, the standard for 'Craft Brewing' has been increased to allow for their continued categorization as a Craft Brewer. Even with the increases, the gulf in capacity between 'craft' and the various major macrobrewers is so large as to render the adjustments to the definition of 'craft' largely benign. The additional standards that a so called 'craft' brewery must restrict the use of cheap adjuncts in their grain bill keeps out the few smaller regional breweries that might otherwise fall under the umbrella. There's no 'craft' in making cheap alcohol water out of rice and corn.
