With your purchase of The Orange Box, you'll get Half-Life 2: Episode Two, Team Fortress 2, and Portal. They're all based on the same graphics engine, but they're all incredibly different games, which ensures that there's something for everyone. You'll also get 2004's Half-Life 2 and 2006's Half-Life 2: Episode One, which is handy if you aren't up to speed with what Gordon Freeman's been doing over the past few years. On the PC, you'll launch each game separately. On the Xbox 360, the game boots up to a menu where you can easily select any of the five games, and quitting out of a game brings you back to the selection menu.
Let's start with new stuff. Episode Two is the continuation of the Half-Life 2 story. It picks up right where Episode One leaves off, with Alyx helping Gordon out of the rubble of a train crash. You've escaped from City 17, which now looks more like a smoking crater in the ground with a huge, swirling portal floating over it. But you aren't safe just yet. You've escaped with information that the Combine very much wants to get back from you, so the chase is on again. Fortunately, you'll do much more than just run in Episode Two. The biggest difference here is that Alyx doesn't directly accompany you through the entire game. You'll split up much more frequently, so, for example, you'll find yourself working your way through antlion nests and crushing antlion grubs all by your lonesome. You'll also negotiate a mine with the help of a vortigaunt who happens to serve as a subtle form of comic relief. It spouts dialogue that plays off of the serious, spiritual tone that most of these aliens take, only applying it to things such as crates full of supplies that just flew down a broken mineshaft and out of reach. These bits alone give Episode Two a much different tone than the previous games, but there are also significant gameplay alterations.
source : Gamespot