Half-Life is a first-person shooter that requires the player to perform combat tasks and puzzle solving to advance through the game. Unlike most of its peers at the time,Half-Life used scripted sequences, such as a Vortigaunt ramming down a door, to advance major plot points. Compared to most first-person shooters of the time, which relied on cut-scene intermissions to detail their plotlines,Half-Life's story is told entirely by means of scripted sequences, keeping the player in control of the first-person viewpoint. In line with this, the game has no cut-scenes, and the player rarely loses the ability to control Gordon, who never speaks and is never actually seen in the game; the player sees "through his eyes" for the entire length of the game. Half-Life has no "levels"; it instead divides the game by chapters, whose titles flash on the screen as the player moves through the game. Progress through the world is continuous, except for breaks for loading.
The game regularly integrates puzzles, such as navigating a maze of conveyor belts, or using nearby boxes to build a small staircase to the next area the player must travel to. Some puzzles involve using the environment to kill an enemy, like turning on a steam valve to spray hot steam at their enemies. There are few "bosses" in the conventional sense, where the player defeats a superior opponent by direct confrontation.
source: wikipedia.org