A memory leak happens when software allocates memory but does not clean it up. In performance testing, you'll may see the operation that creates the 'leak' called over and over again. Memory monitoring tools ('top' in unix or process monitor in windows) will show memory use increase and increase until something breaks - the system will become incredibly slow, etc.
If you can identify the leaky process, performance /testing/'s job is done. It's time for something else, like performance investigation to find the problem code. Profiling tools can help at this point.