Sometimes you download or record a video in a certain format, but in order to edit, import, or share it, you need to convert it. There are many video encoding formats, such as .mp4, .avi, .wmv, .flv, .3gp, etc. Some of these are good for files with small size, some are focused on preserving the high quality of films, so you have to understand your priorities before starting the conversion.
Equally, there are plenty of programs that you can download to convert your videos in a variety of formats. It works best for people who have to do this task on a daily basis and who want to avoid difficulties such as size limits.