The supine bridge is a great exercise for the glutes/butt. It is relatively easy to perform and requires no fancy equipment. While the butt is the primary focus, the supine bridge also works all thigh and hip muscles (e.g. hamstrings, quadriceps, etc), and even the abs. These muscles allow the "bridging" of the hips to take place. Here is how this exercise is done:

Begin in the supine position (on you back) with your knees bent and feet flat on ground. Place the arms by your sides, palms face down. Next, "bridge upward" by pushing through the heels and lifting the hips off the ground. As the hips come up, try to squeeze your gluteus muscles for a few seconds, then slowly lower the hips and repeat the exercise. Be sure to keep your head on the floor at all times.