What is Ad-hoc testing?

Ad hoc testing is concern with the Application Testing without following any rules or test cases.
For Ad hoc testing one should have strong knowledge about the Application.

Describe bottom-up and top-down approaches in Regression Testing.

Bottom-up approach : In this approach testing is conducted from sub module to main module, if the main module is not developed a temporary program called DRIVERS is used to simulate the main module.

Top-down approach : In this approach testing is conducted from main module to sub module. if the sub module is not developed a temporary program called STUB is used for simulate the submodule.

What is the difference between structural and functional testing?

Structural testing is a "white box" testing and it is based on the algorithm or code.

Functional testing is a "black box" (behavioral) testing where the tester verifies the functional specification.