(And what's it good for.)

Related MO questions (with some very nice answers): [examples-of-categorification][1]; [can-we-categorify-the-equation $(1-t)(1+t+t^2+\dots)=1$?][2]; [categorification-request][3].


  [1]: https://mathoverflow.net/questions/43579/examples-of-categorification
  [2]: https://mathoverflow.net/questions/1465/can-we-categorify-the-equation-1-t1-t-t2-1
  [3]: https://mathoverflow.net/questions/55721/categorification-request