What damage is caused when the cambelt snaps on a 2000 citreon Picasso 2.0hdi engine?

It depends on the engine and how it's designed. Often an engine is designed with limited clearance between the valves and the piston. When a valve is opened, it is often in the same space that a piston occupies when it's at TDC. If that's the case, a broken timing belt will result in a destroyed engine. Other engines are not engineered to have that kind of compression so the pistons have a recess for the valves. The first type is called an "interference engine", meaning that the valves and pistons would interfere with each other if they're both trying to occupy the same space. The second is NOT an interference engine. Timing belts are a cheap way to build an engine. GOOD engines use timing chains which do not typically fail until the engine is pretty much worn out.