Select both circles and choose "Combine" under Object menu/shaping (or press Ctrl+L)
Ariel is correct, though I would add that there are several ways to achieve the result, and they don't all work the same way in every situation. Personally, I would "trim" in most cases. There is a back minus front / front minus back, but I really never use them.
What I really came here to note is that sometimes (granted, not all that frequently, but sometimes) I like to use outline weight to achieve the shape, and then use the "Convert Outline to Object" tool. I find it useful in cases of measurement precision, where I know an outline's point size to be a specific measurement in a given unit - an 18pt outline measures exactly .25", 36pt = .5", 54pt = .75", 72pt = 1", once you've run the "Convert Outline to Object", that is.