According to the Institute of Brewing there are four categories as
     follows:

     Large Brewers - Production in excess of 500,000 barrels/year
     Regional Brewers - Production between 15,000 and 500,000 bbl/yr
     Microbrewers - Production less than 15,000 bbl/yr
     Brewpubs - Production for onsite consumption only

     In addition you may see/hear the term pico-brewer which is used to
     describe brewers so small that distribution is limited to pubs and
     bars in their immediate area. To complicate matters their are
     contract brewers. These companies develop a recipe and then "buy"
     excess capacity at a large brewery to have their beer made for them.
     They, then, market and distribute the finished product. Some of these
     can be quite large. The Boston Beer Co., which brews the Sam Adams
     line, is a good example of a large contract brewer.

     To give you a better perspective here are some examples with 1993
     production figures (barrels per year):

     Large Brewers:
          Anheuser-Busch - 93,000,000
          Miller - 49,000,000
          Coors - 25,000,000

     Regional Brewers:
          Boston Beer - 450,000
          Sierra Nevada - 104,325
          Anchor - 92,000
          Pete's - 74,000

     Microbrewers:
          Summit - 10,500
          Celis - 10,500
          Yakima(Grant's) - 8,000

     Brewpubs:
          Wynkoop - 4,200
          Gordon Biersch (No. 3) - 2,700
          Great Lakes - 2,700