What is Stub and Driver ? What functions are used in stubs and driver?

Top down integration testing we have 3 modules A,
B and C. say A and B are ready but C. But A and B calls
functions from C. In this case, to proceed with the testing
a dummy code is written to work as C. This function/code is
called a Stub.

Same way in Bottom up testing Say A, B are ready but C is
not. And C has to return value to A and B. For this a dummy
code is written to return a vale. This is called Driver.