bad struts? warped rotors? bad hub? failing tie rods? bent wheel? could be a number of things. jack it up, and do the 12 and 6, 3 and 9 test. grab the tire from the 12 and 6 positions and try to wobble it up and down. if you get movement, its a bad hub. do the same with 3 and 6, if you get movement, its usually tie rods. in order to tell about bent rims and warped rotors, you normally need a shop to look at them. for the struts, does the car seem more "floaty" than it used to be? push down on the bumper, if the car bounces more than once, you have bad struts.
DISCLAIMER: these are all the driveway methods of checking your car out and are not 100% effective.