The iPhone and iPod touch supports video formats encoded using the MPEG-4 and H.264 codecs; video files encoded in these formats end with .m4v, .mp4 or .mov file extensions. If you have videos in other formats, you need to convert them to a compatible format before you can play them on the iPhone or iPod touch.
A number of solutions are available, including the Videora Video Converters (videora.com), a series of free applications for PCs running the Windows OS. Note that different versions of the program are available for converting videos to the iPhone, iPhone 3G, and iPod touch. A how-to on using Videora is available on the CNet Reviews Web site (reviews.cnet.com/8301-12519_ 7-10050521-49.html?tag=nl.e501)
After converting the video to a compatible format, use iTunes to sync the videos with your iPhone or iPod touch. (A how-to on this was published in the premier issue of iPhone Life: iphonelife.com)
Another popular free solution is Handbrake (handbrake.fr), which is able to directly transcode DVD, HD-DVD, and BlueRay sources.