Prepare yourself for the best FPS, the most confusing puzzler and the most intense multiplayer. This is The Orange Box!
Half Life/Episode One/Episode Two.
Half Life 2 is the sequel to Valve revolutionary and legendary FPS, Half Life. Fear not, because the sequal follows up Half Life is every way you imagined. The graphics are brilliant, sound is awesome and environments are stunning. You are put in the role of Professor Gordon Freeman, he is taken away from from the crowd of City 17 to see Prof. Kleiner. Before you know it, you are running for you life from the alien race known as the Combine. From there you'll find yourself airboating across sewage lakes, fighting through a ruined town full of zombies, speeding across a vast plain and breaking into a fortified prison.
Episode One is the mini-sequel to Half Life 2. There is few gameplay and graphic tweaks, it's more just a few extra levels. It's great nonetheless.
Episode Two is slighltly longer than Episode One. The biggest addition is the fact that it separates your flashlight from your HEV Suit's Aux Power, not much.
Portal:
Possibly the most confusing confusing puzzle game since Tetris, Portal implements a dimension on puzzling. Portal casts you as Chell, an Aperture Science test subject under the control of GLaDOS. You are in possesion of the Aperture Science Handheld Portal Device, a gadget that lets you set a two portals and go through either. It's similar to the portals from Prey, only you get to control them and use them to solve various puzzles. It has the same visual style as Half Life 2 and is semmingly set around the same time.
Team Fortress 2.
The most diverse multiplayer you'll ever play. TF2's main advantage is it's memorable classes. Ranging from the identity changing spy, to the slow moving, room clearing Heavy to the speedy Scout. The game modes are different. There's control, where teams battle for control over command points, similar to those of SW:Battlefront. Then there's Attack/Defend, one team defends command points, while the others attack it, simple.
Overall, this is the deal you'll ever get in gaming. What's left to say? Buy it, perhaps?