For a detailed demonstration of proper deadlift form, see the following video:
Deadlift demonstration from Chris on Vimeo.
A deadlift begins with the barbell on the ground. Step to the barbell, placing feet shoulder width apart under the bar. The middle of the bar should be over the middle of your foot and your knees should just barely be in front of the bar. Your back should have a natural arch*. Grip the bar with both hands and lift it up off the ground. The first few inches will be powered by your quadriceps–your knees started out in front of the bar, and your quads pull the knees backward, straightening them out. From here, you’ll pull the bar up by straightening your your body at the hips. This uses the hip muscles, your strongest muscle group. Set the barbell back on the ground in the reverse of how you picked it up–bend the hips first to allow the barbell to travel below the knees, then bend the knees to set the barbell down.
*To check what a natural arch looks like, stand up straight like you’re at attention in the army. Notice the gentle curve in your lower back. This is the natural arch you should maintain in all barbell lifts. It’s helpful to think of the back as holding while you do the actual lifting with other muscles (in this case your hip muscles).