Shim, engine breather system vents any pressure build up and excessive oil vapour from the engine crankcase. As the pistons move up and down and to a certain extent as crankshaft rotates, any piston blow by (there is some in every engine) starts to pressurise the crankcase. A breather system simply vents this pressure.
In earlier years, venting to atmosphere or a catch tank of some sort was all that's required. However, as emissions regs increased this led to slightly more complicated systems to "re-cylce" the pressure into the induction system.
If it blocks there can be excessive pressure build up leading to oil leaks. A leaking system will simply be messy but may also lead to rough running if it's linked to the induction manifold.
Shim, it won't lead to overheating. The excessive pressure build up starts to push pressure/oil past various oil seals in the engine. The main problem would be leaks pass main crankshaft seals, cam seals, etc. It may even blow a breather pipe off/dip stick out, that sort of thing. Also engine pumping would increase leading to small power loss.
The end result would lead to engine wear/damage as Shane said but you'd have to have absolutely no mechanical sympathy if you let it get that bad. Even you'd notice.:thumbs:
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