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Klaus and Will were right about describing which engines are interference and which are not, but they forgot to say what the terms mean, so in case you don't know, here goes.
Interference refers to the possibility, if the timing belt (or chain, in other makes such as Mercedes) breaks, that the valves might collide with the pistons, doing damage to them. In other words, the interference is that the valves and pistons could get in each other's way!
The timing belt ensures that the valves open and close in synchrony with the piston's up and down motion. When the valves open, they protrude farther into the cylinder closer toward piston, and the piston should not be in its uppermost position at that time. If they're not held in synchrony they could, in an interference engine, collide when the piston moves up toward the open valves.
In a non-interference engine (such as Volvo's redblock, except the unique 16-valve engine), the open valves do not protrude enough to collide with the piston even at its uppermost range of travel -- thus, even if the belt breaks, there cannot be any damage, and the engine just stops running. If you have such an engine and the belt breaks, don't let any unscrupulous mechanic tell you a horror story about engine damage -- it's impossible.
But most engines of different makes, and the white block engines from Volvo, are all interference type, and if the belt breaks, there's going to be an expensive engine repair (or replacement) called for. That's why it's so important to change the belt (and tensioner) at prescribed intervals, to prevent such damage.
Hope this helped.
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