Mark of the Ninja consists of stealth oriented 2D platforming. The character is required to sneak through a series of environments, staying out of sight of guards and performing silent takedowns. Levels are divided into areas of light and darkness; the player character is invisible while in the latter, though he may be given away by any sounds he may make, such as by running, which are represented visually by a rapidly expanding circle. Unusually for a platformer, Mark of the Ninja employs a line of sight mechanic, meaning that enemies which would be invisble to the character are also invisble to the player, though their sounds are represented on screen in the same manner as the player character's.
By approaching enemies undetected, the player character is able to execute a one-hit kill. In order to do this, they are able to extinguish light sources, hide behind objects and pass through narrow grates and shafts which are inaccessible to enemies. If detected, they must evade their enemies and remain out of sight for a set period, after which enemies will return to their original positions and one-hit executions will once more be possible.
Source: Wikipedia, "Mark of the Ninja", available under the CC-BY-SA License.