Barrel aging and craft beer is usually done for 1 of 2 reasons.
One is to extract some flavour properties from the wood or the liquid that was in the barrel previously. The other is to encourage the growth of microflora which might add a souring aspect to the beer. The most common barrels used for barrel aging are going to be bourbon barrels and wine barrels. And as you can guess those will give very different properties. Some of the characteristics of the liquid it self, you know the bourbon or the wine will find its way into the final product of the beer. They are a lot of people who say, and I agree with them that every barrel is different.
But through a general process of trial and error many brewers have really come to some constiency with there barrel aged beers and are quite well known for it. So craft brewers today, are really enjoying experimenting with this new technique to try and derive a lot of very unique and special flavors by using the process of barrel aging in their beer.