Taekwondo Wiki

Taekwondo employs a number of striking surfaces. Among these are:

Striking Surface Korean Term Description
Foot Surfaces
Foot Blade pal nal

This is the outside edge of the foot, along the line from the little-toe to the heel. When striking with the Foot Blade one normally pulls the toes upward as far as possible, and turns the ankle inward as far as possible. Example: the Side Kick strikes with the Foot Blade

Reverse Foot Blade pal deung

This is the inside edge of the foot, along the arch of the foot. Example: the Crescent Kick strikes with the Reverse Foot Blade.

Bottom of Heel tui jook The bottom of the heel is used, for example, in the Back Kick.
Back of Heel twi ggum ch'l The back of the heel is used, for example, in the Hook Kick.
Ball of the Foot ap jook

This is the bottom of the foot, along the base of the toes. Typically when striking with ball of the foot, the toes are pulled upward as far as possible. Examples: (1) the Push Kick is often performed with either the entire bottom of the foot, or with just the Ball of the foot in order to extend your reach. (2) in some forms of taekwondo, the Roundhouse Kick strikes with the Ball of the foot, as can the Front Kick. Of course in both cases the toes must be pulled very far upward to avoid injuring the toes.

Instep, or Top of Foot The top of the foot is is used for example in sport-style Front Kicks and Roundhouse Kicks. The bones on the top of the foot are relatively weak compared to the bottom fo the foot, but kicking with the top of the foot allows you to extend your reach. Kicks made with the top of the foot usually center on the top knuckle of the big toe, the strongest surface on the top of the foot.
Leg Surfaces
Knee moo reup Knee strikes are performed with the top of the knee, where it joins to the thigh, not with the more vulnerable point of the knee. 
Shin chung gang i
Calf chang dan ji
Hand Surfaces
Knuckles, or Forefist The Knuckles, or forefist, are used for common punches.
Backfist The Backfist employs the top of the hand's knuckles rather than the front of the knuckles.
Four Knuckles Instead of closing the fist completely, the fingers are held out and only the first knuckles are bent, thereby presenting the upper set of knuckles as the striking surface. This slightly extends your arm's reach while still providing a strong striking surface. A Four Knuckle Strike is used, for example, in the form Old Koryo.
Three Knuckles, or Chestnut Fist Similar to normal fist except that first three knuckles are pushed outward slightly with thumb.
Single Knuckle, or Knuckle Fist Similar to a normal fist except the middle knuckle is extended. This striking surface is used, for example, in the form Cheonkwon where it is used for a Double Knuckle Uppercut.
Knifehand, or Hand Blade son nal deung The Knifehand, also called the Hand Blade is the edge of the hand on the pinky-side. The fingers are slightly bent so that the hand is shaped "like a cobra's head."
Ridgehand The Ridgehand is the end of the hand along the thumb side. The striking surface is not the thumb itself. Rather, the thumb is tucked tight into the hand so that the striking surface is the side of the first knuckle of the index finger.
Hammerfist The Hammerfist is the muscled bottom of the hand.
Fingertip son gguet For Fingertip Thrusts, the hand is held like a Knifehand, but the striking surface is the pointed tips of the fingers. Usually the target of the fingertips is soft fleshy areas like the opponent's throat, but some people break boards using their fingertips!
Palmheel pa t'ang son The striking surface is the muscled base of the palm, as with a Palmheel Block.
Arc-hand, or Tiger Claw A strike using the space between the index finger and thumb. Fingers are made rigid, and the attack is usually directed towards the throat. Used, for example, in an Arc-hand Strike
Thumb Eomji The hand is made into a fist with the thumb slightly extended. The striking surface is the bottom of the thumb.
Scissor Fingers A fist in which forefinger and middle finger is extended out as if to dig someone's eye. Similar to Pincer hand except that the forefinger and middle finger is extended outwards.
Arm Surfaces
Back of the Wrist, or Eagle Strike In an Eagle Strike, the fingers all touch together, and the hand is pointed down, exposing the top of the wrist, which is then swung upward to strike the underside of the jaw.
Inner Forearm This is the thumb-side of the forearm. Used for example in an Inner Forearm Outside Block.
Outer Forearm This is the side of the pinky-side forearm. This surface is used, for example, in the Middle Block (i.e., Inward Block).
Elbow p'al goop The striking surface is the based of the elbow, where it connects to the forearm, not the pointed portion of the elbow, which is too fragile. This surface is used, for example, in an Elbow Strike.