What is functional testing?

Functional testing is test working of web elements; drop down, buttons, check box or radio button, and features of an application. Functional testing is categorized into various types such as unit test, integration tests, GUI tests, localization tests, usability tests, regression tests, system tests.
Example: on clicking print button, a pop menu should display showing existing page and print configuration window, with print button. On clicking on print button, contents such as text, images or spreadsheets on selected page should be sent for printing.