Functional testing : Testing the functionality of a circuit board, rather than testing all the possible  states the board can be put in. This can either be done by inserting the board in its intended location, or by mounting the board in a functional tester.

Functional tester : A test bed that can simulate almost any digital environment. The circuit board to be tested is mounted on the test bed in the same way that it would be mounted in its intended location. The functional tester then simulates the board's normal environment by feeding it, in real time, the same type of digital and analog signals that it would normally receive.