Wear On Both Edges: UNDER INFLATION
If a tire looks like this, it may be under inflated. The worst enemy a tire can have is too little inflation pressure. Under inflation reduces treadlife through increased tread wear on the outside edges (or shoulders) of the tire. It also generates excessive heat which reduces tire durability. Finally, it reduces fuel economy through increased rolling resistance (soft tires makes your vehicle work harder). Check your tires regularly for proper inflation. Abnormal tire wear may also be due to misalignment or mechanical problems.

Wear In Center: OVER INFLATION
When a tire is over inflated, the center of the tread bears most of the load and wears out faster than the outside edges. Uneven wear reduces the useful life of a tire. Check your tires regularly for proper inflation. Abnormal tire wear may also be due to misalignment or mechanical problems.

Cups or Dips in the tread: WORN PARTS
Cupping (also called dipping or scalloping) is most common on front tires, though rear tires can cup as well. It may be a sign that wheels are out of balance or that suspension or steering system parts are worn out.

Wear on one side: MISALIGNMENT
Or
Sawtooth edges: MISALIGNMENT
Do the edges of the tread take on a sawtooth or feathered appearance? This is caused by erratic scrubbing against the road. The solution is toe-in or toe-out alignment correction.

see also what everyone should know about wheel alignment.