Car Bibles : The Fuel And Engine
Interference vs. non-interference engines
In an interference engine, the piston will hit the valve if it’s open and mis-timed.
It’s worth mentioning the two sub-types of 4 stroke engine at this point.
Because the valves always open inwards, into the combustion chamber, they take up some space at the top of the chamber.
In an interference engine, the position of the piston at the top of its stroke will occupy the same physical space that the open valves do whilst the piston is at the bottom of its stroke.
It’s important to know if your engine is an interference engine because if the timing belt breaks, at least one set of valves will stop in the open position and the momentum of the engine will ram the piston in that cylinder up into the valves requiring a very expensive engine repair or replacement.
Source: Car Bibles : The Fuel and Engine