Online videos, bluray, DVDs, computer videos, there are plenty of situations where videos are used.. and therefore a huge list of formats that exist.. and codecs.
Ok what's that? Well, a video is much more than an image or audio file.. it contains both. A codec is a mathematical algorithm used to store and compress audio and video streams in a file.. Since some video formats allow their author to choose the codecs for video and audio, you can find a two video in the same format, that uses different codecs.
So it makes your life more complicate. If you want to be able to "just open any video", the best advise we can give is : install and use VLC media player or any other player that includes a bunch of codecs, otherwise you will sometime need to find and install codecs to play a video file, which can be complicate.
Video file formats
Video playing softwares
VLC media player