:)
A heatsink is a finned bit of metal attached to anything that gets hot to increase the surface area available for cooling... see pictures below for a selection of examples as they can be virtually any size or shape....
http://www.google.com...b=wi&biw=1366&bih=541
if the fins get clogged with dust and fluff (as they often do) the cooling is seriously reduced and things start to overheat.
And yes, even with the power off, it's not too bad with a vacuum cleaner, (high pressure air hoses are worse) but it's still worth holding the fans so they can't spin.