It sounds liked warped rotors, but you should have it checked ASAP.
If it IS warped rotors, you need to figure out the cause of the warpage - do you ride the brake (i.e. drive with 2 feet)? If so then that is most likely causing the warpage - the brakes are overheating. The other culprit may be if you had the brake pads changed, but the rotors were cut or resurfaced. This is a common practice, but it only saves you a few bucks, and warped rotors can result.
Here's how it works when you apply the brakes the rotor (the shiny disc you can see) is squeezed by the brake pads (held by the brake caliper - the small fist sized pod attached to the rotor) Brake pads in todays cars are of a higher metallic content then those of the past - therefore a harder brake pad compound. This wears down the rotor almost as much as it wears down the pads. When it comes time to replace the pads, the rotors are re-surfaced or replaced - if they are re-surfaced (ground smooth and level) and the thickness of the rotors is in spec they say they can be re-used. This doesn't take into account how much of the rotor material was lost from when they were new, and it now has far less material to dissipate heat, so from a safety minimum perspective - yeah they'll still work. The problem is there is a much better chance that you will warp the rotors with excessive heat build up.
Good luck with your issue and hope it gets resolved.
Like it has been said on here already it is most likely your rotors being warped. How long has it been since your last brake inspection or brake change? Here is what I would do. Go into a local shop, the one I work at is a nationwide company, PepBoys if there is one of these I know they do FREE brake checks. They will tell you what they think it iswrong. If you are tight for money ask to talk to the mechanic, not the person writing up your ticket the one actually working on your car, they will tell you what you absolutely have to have and what might be able to wait. From the sounds of it I would get the Brake pads changed and the rotors REPLACED not resurfaced. If you have not had you car long or if you had the brakes done in the past few months to a year take it back to where you last had it done and see if there is a warrenty. This could save you a lot of money. Also I do not know how long on average you keep your vehicles, but if you like to keep them around awhile, PepBoys has a brake package that if you have the work done there they will replace the pads for free the NEXT time you have them done and all you will pay for is labor sometimes that is up to half off! It can help you out but only if you keep your cars around for a few years. 
Hope this info helps. And BTW I am not trying to push Pepboys on you I just know what we offer, I do however recommened looking into several shops if you think you have the time. Check the mechanics background ask to see if they are certified, ASE certified is a definate plus, not absolutely nessacary but a plus. 
Best of luck to you.
