What is file extension?

A file extension means what kind of file you're working with, like Word Document or a photo.

There are lots of different file extensions. Literally, it's the ".jpeg" or ".doc" ending on the name of the file, which you might not see unless you right click on a file and look at "properties." It includes the dot, so you say "dot jay-peg" or "dot dock"

Pictures are usually ".jpeg" ".gif" or ".bmp"
Word documents are ".doc" or ".docx"
Adobe Reader files are ".pdf"
Powerpoints are ".ppt" ".ppv" or ".ppx"
Executable files are ".exe"

If you want to know more, google "file extensions"