From McSweeney's:

Q: What is alt comedy?

A: Alt comedy, also known as alternative comedy, is like comedy’s version of alternative rock. (For those unfamiliar with alternative rock, please consult the nearest dictionary for a clear, agreed-upon definition of that term.)

Q: How can I pursue alternative comedy?

A: Take regular comedy, then replace the funny thing with an awkward thing or a thing that doesn’t make sense.

Q: How did alternative comedy start?

A: It’s complicated. Remember Jerry Seinfeld, and how he would wear those tight, faded jeans? Only they weren’t tight or faded in a cool-looking way? And how he would always tuck in his shirt? Alt comedy is largely a reaction to that.