Platforms

Arcade, Super Nintendo, Sega Genesis, Sega CD, Game Gear, Master System, Game Boy, PC.
Also available on the Mortal Kombat: Deception Collector's Edition, Mortal Kombat TV Games, Midway Arcade Treasures: Extended Play for PlayStation Portable, and Midway Arcade Treasures: Deluxe Edition for PC.

Release Dates:

Arcade: October 8th, 1992.
Super Nintendo: (Mortal Monday) September 13th, 1993.
PC: May 25th, 1994.
Sega Genesis: (Mortal Monday) September 13th, 1993.
Sega CD: May 26th, 1994.
Sega Master System: 1993.
Game Gear: (Mortal Monday) September 13th, 1993.
Game Boy: (Mortal Monday) September 13th, 1993.

Playable Kombatants:

Arcade, Super Nintendo, PC, Sega Genesis, Sega CD: 7: See Select Screen.
Sega Master System, Game Gear: 6: Same as arcade minus Kano.
Game Boy: 6: Same as arcade minus Johnny Cage.

Hidden (Non-Playable) Kombatants:

All systems minus Game Boy: Reptile.

Hidden Playable Kombatants:

Game Boy: Goro.

Bosses:

Sub-Boss: Goro.
Boss: Shang Tsung.

Finishers:

All: Each kombatant has 1 Fatality.
Super Nintendo, Game Boy: On these platforms, each kombatant has an awesome censored Fatality.

Goro lives...

A 2,000 year old half human dragon Goro remains undefeated for the past 500 years. He won the title of grand champion by defeating Kung Lao, a shaolin fighting monk. It was during this period that the tournament fell into Shang Tsung's hands and was corrupted.

Sega Genesis

The word 'code' has many different definitions. The shaolin martial arts tournament is governed by a system of rules of conduct
...an ethical code.

The combatants respect each other as warriors, no matter what degree of hatred they have for one another
...a code of honor.

Another type of code could be defined as an arbitrary system of symbols or letters for transmitting messages
...a secret code.

Mortal Kombat(tm) adheres to many codes, but does it contain one?

PC

Attention:
The imperial court of immortals invites warriors of all styles to compete in a tournament of Mortal Kombat.

The contest will be held on the island fortress of Shang Tsung located in the center of the lost sea.

Grand Prize:
Your continued existence.

Secret Kombatants:

Reptile: To fight against Reptile, you must follow his clues: Fight in The Pit, when shadows fly across the moon, fight without pressing Block and beating your opponent with Double Flawless Victory and a Fatality.
(Game Boy Only) Goro: After you beat the game, hold Diagonal Up-Left, Select, A on the game boy, until the end of the credits. After the credits finish, it should announce "Goro Lives" with this picture, then you start the ladder as goro, He has a stomp move which I think is just up and he jumps forward, and he has a punch and throw. No fatality.