[RedDevil]

Hi Fat Charlie, read the article Freebeard linked to, or Wikipedia.
It is deceitful, as if designed to put you on the wrong leg.

Remember, the car does not move.
Whatever you choose first has 1/3 chance of being right, no matter what happens next.
The only other possibility is the third door, the one the host did not open. So that has the combined chance of both remaining doors, as one of them was taken out by the host; not at random but deliberately revealing (the) one without a car.

If the host did open one wrong door before you had to make your first choice, then it would be 50/50 between the remaining doors.
But he does so after you've chosen, which means the chance the car is behind one of the remaining doors is 2/3.
And it is still 2/3 when he opens one of them.

It all looks so simple that you may be tempted to not think it through.
So simple, no need to read all this... right?
And it is simple - but you can easily make a wrong turn in thinking it out.

Read the article Freebeard linked to.
Once you get it, you cannot believe you ever missed it.
Swapping gives you a 2/3 chance of winning, trust me.