Awesome additions!
You play as Gordon Freeman once again and Alyx is there to help you out on your journey to escape City 17. Assuming you've played Half Life 2 you'll know what to expect. You have the same weapons in this one as before but the only thing missing in this episode is the use of vehicles. You journey through the game on foot. Other than that you still have the gravity gun at your disposal and some neat ways to use it. Takes about 7 hours to finish.
EPISODE TWO:
Ah episode two is the piece de resistance. They upped the graphics on this one using shaders to enhance the visual effects. Also it is longer than the first by about 3 hours and you get to drive a vehicle, woohoo. There is a ton of fighting. They make you work your ass off in this game, but its well worth it. Also you get to explore some underground tunnels where some weird creatures burrough and journey into the forest. There is also a tragedy but that's all I'm going to say about that.
These both are just beautifull, immersive, awesome games. :)