whats a gasket on a car?

Whats a gasket (like in car kind of gasket)

An automotive gasket prevents leaks, the application is very diverse.

 
A gasket (on anything) is a piece of something that is placed between two parts of something when they are being connected together. As an example, in a connection between two pipes there might be a gasket inserted at the connection which will have a whole in it that allows fluids to pass through it within the pipes but prevents exterior leaks at the connection.

As mentioned above, they are leak preventers/seals. Some are composed of treated paper, cork and rubber, vulcanized cork/rubber/steel, steel, plastic, silicone and 'formagasket' in a tube that is put on between parts then assembled.

There are: head, intake manifold, exhaust (although most engines with turbos do not use a gasket between the head and exhaust manifold = machined steel on steel), valve cover, valley pan(some applications it's also called the intake),oil pan, water pump, thermostat, timing cover, carburetor/EFI body, fuel pump(mechanical, electric, or injector), o-rings that are nitrile or rubber, oil/grease seals at each wheel, cups that are at the end of disc brake pistons or brake cylinders at each wheel, master brake cylinder, door, trunk and hood weatherstripping/seals, between the door and the door panel that are usually large preformed and waterproof-treated paper), there has got to be more I missed like the ones inside the ABS brake controller, and air lines on trucks and self-leveling car suspensions(usually o-rings or compression fittings), then there is THE gasket that nobody wants blown and that is the one between our ears!