Cumbia is Colombian folkloric dance, on the clave beat just like salsa, but a little earthier, heavier down beat, and marked by the tic-tic-tic, tic-tic-tic of a wood block. It's very hard to describe, it's one of those things that once you've heard it you know what it is. There are cumbia licks and interludes in salsa quite a bit, especially Colombian salsa. It started out in the mountains of Colombia, and in it's pure form is basically Colombian country music. But, in it's modern form can be just as tasty and appropriate as salsa at a club.
I don't have my cd's in front of me right now, but there are three songs I can think of that are excellent examples of cumbia that you should ask your dj for next time...
1) "Golpe de Folklore" by Grupo Niche. It starts out as salsa, EXCELLENT salsa as usual from Niche, but the middle part is Cumbia, and you'll know what it is when you hear it.
2) "Canoa Ranchaa" by Grupo Niche again, the whole song is Cumbia.
3) "La Gota Fria" by Colombian superstar Carlos Vives, one of the most recognizable songs in all of Latin America, has a lot of Colombian valenatto influences in it but is basically a cumbia, also a "guaranteed floor packer" for all latin djs.
I'm rambling, but I have to say one more thing about Cumbia. It is heavily popular in Mexico, but there is a WORLD of difference between Colombian Cumbia (the original) and Mexican Cumbia. They're not even in the same universe as far as I'm concerned. A lot of people who are turned off to cumbia had the misfortune of hearing Mexican cumbia first and thinking that's all there is to it. That would be like hearing J. Lo and thinking that's all there was to salsa. Not!