how to start a video from the terminal?

The syntax to open any file in its default application is

xdg-open <file_name>

If you want to have some fun and play the video in the terminal itself, install mplayer (sudo apt-get install mplayer) and run

mplayer -vo caca <movie_file>

It doesn't run directly in the terminal window, but it does display in ASCII characters.

In general you start any command from a terminal by entering the command.

So to edit a file with gedit

gedit file_to_edit

An .avi is no different, just a different command

banshee your.avi

You may use any media player you wish.

the only thing is an avi itself is a container, so you may need to install some codecs. codecs are the tools to play mp3 and other audio visual files.

this link may help

LinuxCommand