Functional Testing sometimes called black box testing because no knowledge of the internal logic of the system is used to develop testcases.for example ,if a certain function key should produce a specific result when pressed, a functional tet validates this expectation by pressing the function key and observing the result.when conducting functional tests, you will use validation techniques almost exclusively.

Functional testing advantages:

simulates actual system usage.

makes no system structure assumptions.

Disadvantages

Potential of missing logical errors in software

possibility of redundant testing.

Functionality Testing:

Its part of Functional testing. Giving the input and checking the output. Testing the application against specifications.