i want to download some movies but i dont know what type of file i should download(ex: m4, avi etc.)for my ipod video. please help.

should be with the extension mp4

mpeg 4 is what iPod needs. There's also H.264 which is a variation of mpeg4. The extension is either .mpg or .mp4 for mpeg 4. See How to transfer to my iPod (& iTunes) - for video. iPod may also have some resolution maximum limitations also. The joined audio format may also be require to be within a range of formats.