From the website:
"Currently, the best format for such use is FLV (Flash Video), as almost all browsers will display it natively without need of installing extra plugins. It is for example the format used in Google Video."
You can control the size and bit rate (quality) so you can create a widely compatible video, but still make it high quality.
"The most important thing in a man is not what he knows, but what he is." -- Narciso Yepes