Half-Life 2 is a source engine game designed as a story extension of Half-Life. The Half-Life series is about the downfall of earth due to an experiment gone wrong which opened a portal to another universe called Xen. Through this portal all of the inhabitants of this universe was informed of the presence of earth, good or bad.
This is the Half-Life 2 logo on the right. Half-Life 2 is also typically shortened to hl2.
Story
On the left here is a catapult made in the game with the tools and props provided in the game.