Ipod Nano and Classic supports H.264 video, MP4 video and .mov video with a maxmium screen resolution of 640 X 480 pixels. Your ipod will not play videos on other format and you will not be able to add them to your iTunes Library also. I have attached a video which I have converted from YouTube to iPod and I used it on my iPod Nano.
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You can use third party software to convert your other videos format to iPod format.