Basic Moves:

Moves that require only a button press:

HIGH PUNCH - Produces a punch to the face. Many can be strung together in succession by rapidly pressing the button.
LOW PUNCH - Your character will punch the opponent's midriff. Many can be strung together in succession by rapidly pressing the button.
HIGH KICK - Smacks your opponent in the face with a powerful kick
LOW KICK - Not as powerful as the high kick but a little faster to recover, kicks the opponent in the abs.
BLOCK - Blocks attacks

Combination Moves:

Moves that require a button to be pressed simultaneously with a direction:

Jump Kick - Jump + either KICK button.
Jump Punch - Jump + either PUNCH button.
Pressing up plus kick or punch will result in your character punching or kicking in midair without moving in a direction. If jumping towards or away from the opponent, pressing punch or kick will cause your characrter to extend their limb a little further to hit the opponent and will cause more damage. A Jump Kick is more powerful than a Jump Punch, but Jump Pucnhes are better for winning aerial battles.
Uppercut - Down + either PUNCH button.
The most damaging move in the game - will cause about 24.5% of your opponents health to be decreased in one hit, sending them soaring into the air.
Sweep - Back + LOW KICK
Sweeping knocks your opponent onto their back by tripping them over.
Roundhouse - Back + HIGH KICK
Your character spins around and gives your opponent a heavy kick to the side of the head, sending them sprawling back. Like the Uppercut, this move is also very damaging.
Weak Kick - Down + LOW KICK
Not really useful unless your opponent is ducking and avoiding your other attacks...weak and slow.
Throw - Forward + LOW PUNCH
Must be standing right next to your opponent. Different characters throw in different ways, but all throws do good damage. Throws cannot be blocked normally.
Block Throw - BLOCK + diagonally down + away
Pressing down and away while blocking will prevent your opponent from executing a throw.
Death Punch - Standing punch when opponent is jumping towards you.
Timing is the key...this move is pretty difficult to pull off, but when you can do it, it is VERY damaging.
Power Hit - Forward + HIGH PUNCH
Some characters elbow, some headbutt, but the basic premise is the same - a strong hit to the opponent in close quarters.

Special Moves:

Moves specific to each character that must be done with a combination of buttons and directions.

Kano:
Knife Throw - Hold BLOCK, then back, forward while holding block (14.8% damage)
Kano throws a big knife at the opponent
Cannonball - 360 degrees toward opponent (18% damage)
Kano rolls into a ball and launches himself at the opponent, sending them flying across the screen. It helps to hold block as you roll the joystick/gamepad. This move will leave him vulnerable to be hit if blocked.

Fatality (when opponent is depleted of health) - Back, Down, Forward, LOW PUNCH
Kano rips out his opponent's heart and holds it high in the air (Stand close)

Johnny Cage:
Green Flame - Back, forward, LOW PUNCH (14.8% damage)
Cage throws a ball of green flame at the opponet
Shadow Kick - Back, forward, LOW KICK (18% damage)
Cage produces a powerful kick, sliding across the screen leaving a shadow behind him
Groin Punch - BLOCK + LOW PUNCH (21.1% damage)
Cage does the splits and punches the opponent where it hurts! Does not work on Sonya for obvious reasons

Fatality (when opponent is depleted of health) - Forward, Forward, Forward, HIGH PUNCH
Cage gives the opponent a mighty uppercut that decapitates them (stand close)

Sub-Zero:
Ice Blast - Down, Forward, LOW PUNCH (0% damage)
This move will freeze the opponent temporarily, leaving them vulnerable to attack.
Slide - Back + LOW PUNCH + BLOCK + LOW KICK (7% damage)
A good move to slide under an opponent's projectile or to hit the opponent after a jump kick or Uppercut before they hit the ground.

Fatality - Forward, down, forward, HIGH PUNCH
Sub-Zero grabs the opponent's head and rips it from their body, leaving the spine dangling beneath it. (Stand close)

Liu Kang:
Fireball - Forward, Forward, HIGH PUNCH (14.8% damage)
Liu Kang shoots a Fireball at the opponent.
Flying Kick - Forward, Forward, HIGH KICK (18% damage)
Liu Kang laucnhes himself across the screen and hits the opponent hard with a flying kick.

Fatality - 360 degrees towards opponent
Liu Kang Cartwheels then Uppercuts the opponent high into the air. (Stand at Sweep Distance)

Scorpion:
Spear - Back, Back, LOW PUNCH (4.6% damage)
Scorpion throws a spear with a rope attached to it at the opponent, stabbing them in the chest. He then pulls on the rope to bring the opponent towards him. The opponent is then vulnerable to attack.
Teleport Punch - Down, Back, HIGH PUNCH (18% damage)
Scorpion jumps off the edge of the screen to reappear on the other side, punching the opponent in the chest. Can be done in the air. Does not work when standing on the edge of the battle arena.

Fatality - Up, Up (holding BLOCK helps to avoid jumping)
Scorpion pulls off his mask to reveal a skull. He then breathes fire on the opponent, ingulfing them in flames until only their skeleton remains.

Raiden:
Lightning Blast - Down, Forward, LOW PUNCH (14.8% damage)
Raiden shoots a blast of lightning at the opponent
Torpedo Launch - Back, Back, Forward (18% damage)
Raiden dives Horizontally at the opponent, carrying them for a short distance before smashing them into the ground or the edge of the battle arena. If this move is blocked, it leaves him vulnerable to a projectile attack.
Teleport - Down, Up (0% damage)
Raiden disintegrates into electricity and reappears on the opposite side of the opponent

Fatality - Forward, back, back, back, HIGH PUNCH
Raiden fires bolts of electricity into the opponent's head until it explodes (Stand at Sweep Distance)

Sonya:
Force Wave - LOW PUNCH, back, back, LOW PUNCH (14.8% damage)
Sonya fires a wave of pink energy rings at the opponent
Teleport Punch - Forward, Back, HIGH PUNCH (16.4% damage)
Sonya Flies across the top of the screen with her fist extended, hitting anyone in the way. Will only hit opponents while they are in the air.
Leg Grab - Down + LOW PUNCH + BLOCK + LOW KICK (24.2% damage)
Sonya does a hand stand and grabs the opponent with her legs, before flipping them over her and slamming them into the ground. Must be close to opponent to hit them, and leaves her vulnerable to attack if blocked (must be duck-blocked) or avoided.

Fatality - Forward, forward, back, back, BLOCK
Sonya blows a kiss at the opponent (which is actually a fireball) that burns the opponent until only their skeleton remains (Stand outside Sweep Distance)

Secrets:

Mortal Kombat only had one secret - and that was the hidden character Reptile. This was a Ninja dressed in a green costume that had the moves for both Sub-Zero AND Scorpion.

To fight against Reptile (he was not playable) you must be on the Pit Stage. Before the fight begins, you need to see a witch or something similar fly in front of the moon, and this will happen totally randomly. If you see the witch, you must win the match DOUBLE FLAWLESS and finish your opponent with a fatality, without using the block button (except for Scorpion and Sonya's fatality).
Note: It is NOT considered a fataltity in the original Mortal Kombat if you Uppercut your opponent into The Pit.

If you can successfully pull that off, you will be whisked away to fight against Reptile at the bottom of the Pit! Good Luck!