It's getting harder to tell because of changes in tire and suspension designs. But as a rule, "normal" wear is when the tread wears evenly across the entire surface of the tire. The edges and center sections of the tread wear down at approximately equal rates, and no bumpy, directional, feathered or cupped wear patterns develop on the tread.
What's more, both front tires and both rear tires wear at approximately the same rate. Front and rear tires usually wear at different rates depending on which end is doing the driving. The front tires on front-wheel drive cars and minivans, for example, wear at a much faster rate than the rear tires. The rear tires on rear-wheel drive performance cars or vehicles driven by someone with a heavy foot also tend to wear out much faster than the ones up front. But this is "normal" for the way in which the vehicle is driven.
Heavy shoulder wear on the tires is also considered "normal" if a vehicle is driven hard around corners. Rapid shoulder wear on the front tires is also "normal" on some trucks and minivans because of the steering geometry of the vehicle. The front wheels are supposed to "toe out" with respect to one another when they are turned to either side to compensate for the different path the inside and outside wheels follow when turning a corner. Some vehicles are better designed than others to accomplish this. Those that aren't tend to produce more shoulder wear than those that do. Rotating your tires frequently (every 8,000 miles or so) can help to equalize this kind of wear between tires.
"Abnormal" tire wear is any type of wear that results from a suspension or alignment problem, an internal tire fault, or driving on underinflated or overinflated tires.
Abnormal wear would be where the inside or outside edge or shoulder of the tire shows extreme wear, but the rest of the tread shows little wear. This is called "camber" wear and results from the tire leaning in or out (it should be straight up and down when rolling down the road). Camber wear can be caused by suspension misalignment, a bent strut, a mislocated strut tower (often the result of unrepaired collision damage), a weak or broken spring, a bent spindle, or collapsed or damaged control arm bushings.
The suspension should be inspected for worn or damaged parts, and an alignment check performed to determine what needs to be fixed to correct the problem.
If the tread develops a feathered or directional wear pattern where the tread feels smooth when you run your hand across it one way, but feels rough when you rub it in the opposite direction, you have a "toe" wear problem. Toe refers to the parallelism between the wheels as the roll down the road. If the wheels are toed in or out with respect to one another, the tread will scuff and develop a feathered wear pattern. This may be due to toe misalignment, worn tie rod ends, worn idler arms, bent steering linkage or bent steering arms. As with camber wear, the suspension should be inspected, and the alignment checked to determine what's causing the problem.
A "cupped" wear pattern on the tires can be caused by a wheel and tire that are out of balance or by weak shock absorbers or struts. This type of wear occurs because the wheel bounces up and down as it rolls down the highway. The cure here is to have the wheel balanced or replace the worn shocks or struts.
If the center of the tread is worn more than the shoulders, it may be the result of overinflation. You're putting too much air in your tires, causing them to bulge out in the center and wear unevenly. Refer to the recommended inflation pressures in your owner's manual or on the tire inflation decal in the glovebox or door jamb.
If the shoulders of a tire are worn more than the center, it may mean the tire doesn't have enough air in it. Underinflation shifts the weight carried by the tire to the edges of the tread causing the shoulders to wear more than the center. As with overinflation, refer to the recommended inflation pressure for your vehicle.
NOTE: As mentioned earlier, heavy shoulder wear can also be caused by hard driving, especially on winding or curving roads. In this case, nothing abnormal is indicated, and the only correction that's needed is a change in your driving habits.
Some low profile performance tires have a tendency to develop what's called a "heel and toe" wear pattern if they are not rotated every 5,000 to 8,000 miles. This is caused by tread flex and the belt design of the tires. If tires with this kind of wear tendency are not rotated, the tread may develop a washboard wear pattern that causes annoying vibrations and/or noise at speeds above about 40 mph. Once the wear pattern is established, it may be too late to reverse it by rotating the tires. Replacing the tires (and switching to a brand or design that is less "quirky") may be the only way to cure this kind of wear problem.