Just wanted to add...especially for lurkers...the clicker is used for dogs because it is a novel, consistent sound. It doesn't HAVE to be a click. With different animals, they use different tones or beeps, flashes of light (I've seen this one for fish)...anything can be a marker. I like the reference to calling it "marker training." I guess, technically, you could even use a hand signal...though that would have the distinct disadvantage of only working when the dog is looking at you.
I have Karen Pryor's Getting Started: Clicker Training for Dogs which is a great basic intro. Not heavy on theory, but breaks down some basic commands and gives the clicker twist on them.
I find that I naturally mix voice and clicks. I do Yesssss and a click sometimes...especially when I am particularly happy with what I'm seeing. It isn't perfect as far as conditioning theory goes, but it works for us. Murphy pretty much knows that a click means treat as well as yesss. Just like he comes running for either "cookie" or "treat".