What is a file format?
A file format is a particular way to encode information for
storage in a computer file.
There will usually be a suffix at the end of a file to tell you
what kind of file format it is- for images, .jpg, .png, .gif,
etc.
Some video file format types are- .wmv, .mp4 (.mpg4),
.flv...
If you don't see a suffix like that at the end of your file then
right click and hit properties, and it should tell you in the file
description.