File format understanding

I'm a newbie so here goes......I use Imovie a lot to create projects of audio visual topics of interest. I import media from various sources like tv recordings and Handbraked items ect. I see extensions like m4v, mpg, mp4, mpg pop up with no idea of where when or what.

Can anyone give me a simple breakdown of this please? I don't really understand the language of any research i found.

simsaladimbamba help source

Thank you for the video link you gave me as a help source which I've only just seen because I had problems getting back to MacRumours...then gave up due to home office upheaval.
The video is excellent and my understanding has grown as a result. I'm not a whizz kid so I like the speaker and his slow but clear presentation.......had to watch & listen several times and feel a lot happier now.

Thanks again!