ABAP Unit is a test tool integrated in the ABAP runtime environment that you can use for executing unit tests- that is, for checking the functions of code sections in a program.
ABAP Unit - Overview
ABAP Unit and Test-Driven Development
Creating ABAP Unit Tests
Executing ABAP Unit Tests
Evaluating ABAP Unit Tests
ABAP Unit Browser
ABAP Unit - Example
ABAP Unit - Terminology