Android supports H.263, H.264 and MPEG4 video formats. Various file extensions types can be a wrapper for some of these, and also all sorts of video files from various sources may play (e.g. AVI), but with mixed results, particularly once the duration is longer and where the intended display size is full screen.
Therefore, presuming that people want to use their Android tablet as a video player, we have created the video below to try and give people optimal results. Other things will work, but if they don't, you can fall back on this.
Unlike Windows where you can go looking for another codec, Android doesn't have this option. Although various video players are available from the Android Market, none of these alter what will play as this is core to the OS, not application specific.
For this video we have used HandBrake, the open-source, multi-threaded cross-platform transcoder.