MOT Certificate
An MOT certificate is an A4 piece of paper proving that your car had been check for road worthiness. When buying a car, always check if the car has a valid MOT certificate carried out by an valid Inspection Authority / Garage. If it doesn't had an MOT certificate the beware that the car maybe a failure and require work before passing.
A certificate is normally valid for a 1 year period, so it is important to see how long the car has left before requiring another MOT Test.
Advisory Notice
It's also a good idea to check if, when the car had it's last test, it was issued an Advisory Notice. This is a minor fault with the car which doesn't fail the car but is advised to resolve.