As per wikipedia.org:
Test automation is the use of software to control the execution of tests, the comparison of actual outcomes to predicted outcomes, the setting up of test preconditions, and other test control and test reporting functions. Commonly, test automation involves automating a manual process already in place that uses a formalized testing process.
The principle of automated testing is that there is a program (which could be a job stream) that runs the program being tested, feeding it the proper input, and checking the output against the output that was expected. Once the test suite is written, no human intervention is needed, either to run the program or to look to see if it worked; the test suite does all that, and somehow indicates whether the program's output was as expected.