Rock is a naturally occurring solid aggregate of minerals. Not to be confused with brick.
In the environment
In most games, rocks are nature's crates. They're used as a way to break up the environment or offer the player a stepping stone to jump across small gaps or in order to reach higher areas. Occasionally, they spill down in a rock slide with devastating consequences to non-player characters and the player alike. True to their form, rocks are generally indestructible, making them an excellent source of cover in an action game, unless, of course, they're in their fluid state--lava.
As a weapon
Some games use rocks as weapons. They can be used as catapults or trebuchets to bombard an area, as seen in Stronghold Crusader or Age of Empires II: The Age of Kings, or just thrown by hand, as the guards do in Assassin's Creed (whenever the player is attempting to scale a building). Several early NES games used rocks as weapons for the main character to throw at enemies, such as Friday the 13th. Another such game that utilized this way of attacking is Chuck Rock, as well as its sequel.
As a character
Rock-like creatures are commonly seen as characters, typically in fantasy games. In the Pokémon franchise, there are several Rock type Pokémon that are actually made out of rock, such as Onix and Golem. In Darksiders, there are large rock-like creatures who block areas, and must be moved using the Earthcaller (a large horn). In the Legend of Zelda franchise, the Gorons are rock-like creatures who live in the mountains and typically engage in strength based activities, i.e. wrestling.
In puzzle games
There are a variety of games which use rocks in puzzle gameplay. In the Xbox Live Indie Game Miner Dig Deep, rocks and precious ore can be collected by digging deeper (hence the title). Nintendo's lesser-known Mario games, Wrecking Crew and its sequel Wrecking Crew '98, rocks (and bricks) are smashed by Mario's hammer. Another lesser known puzzle game, Rock Blast on the Nintendo DS, uses color matched rocks in its gameplay.
Other uses of rock
In many games, rocks are used as resources (sometimes in the form of stone). The Age of Empires franchise has rocks as a resource that must be mined by villagers, and is required for many buildings and soldiers. In Mass Effect and Mass Effect 2, certain types of rock, stone, precious metals, and minerals can be explored and mined on a variety of planets.