HD in Windows XP Media Center edition

So right now I just have Windows XP SP2 (professional) but I was thinking of getting Media Center edition because I play DVDs/movies/music so much I'm getting tired of using mouse and keyboard, and having to switch through folders etc to access my media. Plus the remote and organization of media center intrigues me. Anyway, I was reading about all the features and wasn't clear on it's HD options. Now, it says it supports HD tuners. How does this work? Do they make HD tuner cards for PCs? How does HD television get to your PC? You need an HD source and an HD tuner card I assume, right?

Basically I want to know how I can get HD TV to my PC using media center edition.