Episode 1 are 2 use the same textures, models, and engine as Half Life 2. The only thing new is the story and the enemies. It picks up after the HL2's ending, and goes on until Episode 3. EP3 isn't even out yet, and there are no release date as of today. Each EP's lenght are quite short, and the ending of EP2 will leave you saying, "why the hell hasn't Valve released EP3 yet? I'm dying to know what happens next." I can guarantee you that.
So it's not a totally different game, it's just a continuation.
So it's not a totally different game, it's just a continuation.