iOS devices like the iPhone, iPad and iPod touch are excellent ways to watch videos. Problem is every now and again a video does it play on the device because it is in the wrong format. However, converting to the correct format should not be difficult. 

The formats that work are H.264, MPEG-4, Motion JPEG, these files are identified because they end with MP4, M4V or MOV. Flash video does not play and neither does WMV or AVI. But using a program called Handbrake, which is available free for Windows and Mac will convert these to the correct format. It can be downloaded from here http://handbrake.fr

Below is a video showing you how to do this. It shows you how to do this for a Flash video (FLV) but works the same for AVI and WMV. 

If you are using video editing software then make sure you choose the right output.