There has always been a tension in XCOM concerning how it should be considered and played: is it like an RPG, with characters and a single story that you should be able to finish at some point after you start? Or is it like a strategy game, where you start, learn, start over, learn more, and maybe finally win? Personally, I've always found the latter idea more satisfying. But the most important thing to know about Enemy Within is that it consists of several major additions to the main XCOM campaign instead of being an entirely new entity.
