The human body has many surfaces which can be used to strike an object. The following is a list of the most commonly used, and the good and bad points thereof.

The fist

Probably the most commonly used surface for striking. Depending on it's usage, it can be the most effective. For actual technique on usage, see "How to use a fist." Because the properly used fist has a small surface area on a very dense and hard bone, it inflicts better damage on targets like the nose and throat. The fist is weak, however, against other hard and dense objects such as the teeth and skull. Broken knuckles take a long time and are painful to heal. Best used at a medium-close to medium range.

The forehead (head butt)

Mostly used for a quick attack when hands are either too slow or not an option. The head butt is similar to the fist in that it is very effective against the nose with minimal damage to the user. The head butt is a very quick motion that is most useful in extremely close quarters. The user can also grab the opponent and pull them towards the attack to extend the effective range to medium-close. The head butt, although a great technique in itself, is too risky to use without a hitch. For instance, once your head is forward, it is easy to be pulled off-balance and become vulnerable.

The elbow

Probably the best surface with which to strike, in my humble opinion. The elbow can do a considerable amount of damage to any target, hard (skull) or soft (stomach). The only downside to this surface is that it requires more preparation time to complete effectively. The two variations of elbow strikes are as follows;
(a)Hold elbow out and swing with circular motion into target
(b)When side is to opponent, draw elbow horizontally away from and then immediately quickly towards opponent in a slide motion.
Both do very well against all upper body targets, hard and soft. Best of all, the elbow has few nerve endings and is very dense with a small striking surface. This translates to concentrated damage with almost no repercussions to the user. However, some users may feel vulnerable using only one side of their body. Using this technique leaves the rear side of the body unable to attack. Attack is best used in a medium range.

The palm heel

The palm heel essentially does what a fist does except a bit differently. The palm heel itself is used by pulling the wrist as far back as possible and striking with a punch motion using the lowest portion of the hand. It is more effective than a punch against harder targets like the skull in that it induces less damage for the user. It utilizes a larger striking surface with more meaty "padding" than the fist does. Although not as hard, the palm heel is better than the fist at breaking ribs and noses. Range is medium to medium-long.

The knife edge of the hand

When holding the hand rigid and open, the knife edge of the hand is that of the pinky side going down to the wrist. The knife edge has the marked disadvantage that it is very weak against hard surfaces like the skull. However, this attack is very strong against a soft target, most notably the trapezius muscle or the sternocleidomastoid. The technique, for use at medium to long range, requires a bit of torque to be effective. Hold the hand pinky-side facing upward by the side of the head. Push nearly straight outwards and upon striking, quickly rotate hand pinky-side down. A proper usage can render the victim unconscious or at the very least cause pain and muscle stiffening.

The knees

For proper technique for this close range maneuver, it is probably best to watch a video clip of Muay Thai, which uses the knees quite a lot. Essentially, grab the opponent with hand either on shoulders or neck. Pull opponent toward you and downward while bringing the knee up to strike. The knee is a great technique because it is hard to avoid due to the grab. The knee is so effective against all targets that, even when blocked, it still can cause a great deal of damage. The choice targets include the groin, face, and stomach.

The shin(round kick)

The shin is the proper striking surface for a roundhouse kick or round kick (depending on style) in a combat situation. In sparring, round kicks usually use the top of the foot as an acceptable surface, but this can cause a hyperextension which pulls the tendons over the top of the foot too far and takes a few weeks to recover. (Trust me on this sucks.) Although not as fast as a punch, the round kick reaches nearly twice the length and has at least as much power as a strong punch. The preferred targets are the thighs, ribs, and head. These can be hit with relatively little danger to the user with proper timing. However, one major risk is that if the leg is caught, the user is easy to sweep and take down. To use, lift leg with knee bent straight out to side, swing sidewats into target with ankle and toes pointed all the way forward.

The knife edge of the foot (side kick)

The side kick is by far the most powerful kick available to a fighter. When the foot is pulled back as far as possible with the leg out sideways from the body with the hip turned over (opened up), maximum power is achieved. The kick is somewhat slow to enact but if/when it hits, it is effective against any target. Speed can be sacrificed for power by stepping across your leg into the kick or even using a flying side kick.

The ball of the foot (front kick)

To use the ball of the foot properly, lock the ankle all the way forward with the toes pulled all the way back. Lift the knee up as high as you can, then push straight outward with the ball of the foot forward. The front kick is (power-wise) like a side kick with power traded for speed. Targets are the same as side kick as well as range.