Nothing brings more pangs of doom and gloom like the words "timing belt". If your vehicle has one, it is the single, most costly maintenance item you will have and thankfully most manufacturers only require it around 100k miles!
This belt basically keeps everything in time because most engines today are classified as 'interference' engines. This means that when the valves are all the way open AND the piston is all the way up, they will occupy the same physical area. The timing belt keep everything in time and prevents any collisions. However, if this belt breaks, the pistons will impact the valves and serious engine damage will ensue. Best case, the cylinder head will need replacing and worst case, a whole engine replacement.