Your rotors are warped. If they're too bad, you will need to replace them. if they are fairly new and haven't been resurfaced before, you can probably just have them resurfaced. It's probably just the front ones.
Oh, braking. My rotors have been turned (ie, resurfaced) twice and that's it - next time it will be new rotors. I bought the Gold Series of Wearever at Advanced Auto Parts and I was very pleased. Came with the backing plates and cost about $30 for the front set.