Now if you had just said coworker I could have said "a coworker orks cows", but no.

I would have to say that a colleague is in a more professionally intimate relationship with one than is a co-worker. I also taste (Hi, xara) more of a professional (as in learnéd professional) overtone to colleague. Also, co-worker has more of a feeling of spatial proximity. Someone who works in the next cubicle would be a co-worker regardless of any direct connection in a business sense beyond working for the same employer; a colleague might work in a completely separate building but would be working on the same things, the same project and at an organizational level matching one's own.