mp4, mov, mkv - What does this all mean?

Ever come across a file that won't open and you have no idea what program to use to open it? FILExt is an very handy site that allows you to type in a file extension and lists all programs associated with it. I have been using this site for years and most recently have found it extremely helpful when assisting patrons with mysterious file attachments in their emails. I may not be able to open those attachments for them, but at least I can tell them what program they can use outside the library.