There's a debate going on over at Tandleman's Beer Blog that includes, not for the first time, the question of "craft keg".
I think most CAMRA members sort of know what people mean when they talk about "craft keg" - keg beer that's been brewed with more attention than mass produced lager and therefore tastes better . But an actual definition of what qualifies as "craft keg" and what doesn't is more difficult.
One definition of "craft keg" is keg beer from a microbrewery. In the US, this makes some sense as most of the tasteless beer is produced by big brewers and most of the decent beer by small brewers. But even in the US, there are exceptions: surely, for example, Budweiser American Ale counts as "craft keg" even though it's produced by the world's biggest brewer. On this side of the Atlantic, the picture is completely different. Big brewers not only produce lots of decent cask beer but "craft keg" as well, like the keg version of Fullers ESB in the picture below. Is Draught Guinness, produced by global brewer Diageo, "craft keg"? And if not, why not?
You could also argue that "craft keg" is keg beer that's unpasteurised or unfiltered. But again, most keg beer in the US - including Budweiser lager - is unpasteurised. A more popular definition is to exclude from the "craft" category beer that's been brewed with adjuncts in addition to barley malt, water, hops and yeast. This excludes beers like Budweiser - brewed with rice for a "lighter taste" - but also overlooks the fact that British cask beer since the nineteenth century has included adjuncts such as wheat for head retention, maize flakes for flavour and most importantly brewing sugars to adjust flavour, strength and colour. That's before isinglass is added to make the beer drop bright.
As an exact term "craft keg" seems pretty meaningless to me. Unlike "real ale" (cask or bottle conditioned beer), it's subjective ("keg beer I like the taste of") and the line between it and Carling or Tetley's Smoothflow is an arbitrary one.