a test harness or automated test framework is a collection 
of software and test data configured to test a program unit 
by running it under varying conditions and monitor its 
behavior and outputs. It has two main parts: the test 
execution engine and the test script repository

Test harness is the collection of Drivers and Stubs. 

Drivers are the called functions i.e main programm that 
accepts test data and passes this to the modules to be 
tested. This is manly used for Bottom-up approach.

Stubs are the sub programms or calling functions and mainly 
used for top-down approach.

In testing time we use drivers and stubs on a software 
build is called test harness.it will help for testing to 
conduct sequencially.

Test environment+test bed=test harness
for necessary envronment for executing testcases