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Upcast is the conversion of a derived type to a super-type. This doesn't require any explicit casting:

	Base base = derived;

Downcast is the conversion of a super-type to a derived type. This require explicit casting:

	Vehicle car = new Car();
	Car car2 = (Car)car;

If I remove the casting I get a compilation error.
If the casting is wrong I get a runtime error. For this reason "instanceof" is used before a downcast:

	if(car instanceof Car) {
		// continue...

Any other conversion different than downcast or upcast for objects generate a compile error or a runtime error.

The same rules apply to primitives, with the difference you can't use "instanceof" and you can sometimes convert between inconpatible types with a loss of information:

	int a = (int)15.5f;