Why do DVDs start in the middle? Is it cause the computer remembers or the DVD?

So, if I take a DVD out and put it back in, it starts playing where I left off.

Why does this happen? is it my computer remembering? I dont quite get how that works and I would love to understand my computer better.

This is probably the software remembering where you left off. For a movie DVD, for example, OS X launches a program called DVD Player. If you open its preferences and go to the "Previously Viewed" tab you can control whether that happens or not.

It isn't "probably", as the DVD itself can not be written to. Go into the preferences of the software you are using to watch the DVD and change them to not remember playback position.

When I insert a dvd DVD Player gives me a message asking whether I want it to play the dvd from where I left it, or go back to the beginning. Do you not get this?

I think it's been covered above; but you can - Go to Applications/DVD player without any disk inserted. From the Toolbar, Select Preferences. Then Click on "Previously Viewed". There you are presented with 4 options: "Beginning", "Last position played", "Default playback", and "Always ask".

These are self explanatory. Tick the appropriate box. (The "Last position played" is probably checked at the moment. Change it to what you want).

Ian