What are [coherent](http://en.wikipedia.org/wiki/Coherence_%28physics%29) and incoherent radiation?

I am a mathematician who is self-learning physics. In reading Jackson's electrodynamics and other books, I often hear that radiation is incoherent or coherent. What does this mean? Does it just mean that the phases are the same in coherent radiation and is different in incoherent radiation?