Most of the time a web video is a low file sized video so that it is easy to upload and easy to stream, here are some of the formats that are used for the web:
This is a small file size and good quality option to use and is one of the most popular because it can be played on many devices such as iphone and ipad.
This is a windows file with a large file format, it cannot be played on a mac without the necessary additions installed, if you want your video to be accessible by everyone then an MP4 is a much better format but you still sometimes see .wmv files being used. The other files that you find are .mov, .avi and .flv but in my opinion the best option to use is the MP4 as websites such as Youtube and Vimeo use these because they give you the best results. When making an MP4 you will also have settings that you can play around with to give you the best result.