High-definition video comes in 720p and 1080p. The numbers stand for the number of scan lines in the image, or the vertical resolution in pixels. The “p” indicates that the image is progressively scanned. In the case of 720p, the progressive scanning reduces flicker, provides good sharpness, and provides an image that is close in quality to 1080i. The “i” stands for interlaced, which is how the image is provided on a standard television. Each “frame” in an interlaced image requires that the screen be painted twice. 1080p is for very large screens.
The minimum needed to play the video is an XP PC with Media Player 9, 2.4 GHz CPU, and a 1024 x 768 screen. For 1080p, which is overkill on most PC screens, you'll need a high-end XP machine with Media Player 10 or a dual-core Vista Machine with Media Player 11 and a 1920 x 1440 screen.
A good video to try first is “Robotica” because it’s the smallest download. (16 MB for 720p and 20 MB for 1080p.)