Everest allows you to stress test your CPU, Memory and hard drives. It uses the benchmarking tests, such as the CPU PhotoWorxx and FPU Mandel, and repeats them over and over, until you stop it. Generally, a stress test stresses more than just one piece of hardware at a time, and so does this. Each of the tests will overlap each other, to give a more accurate result.
Prime95 does not test your hard drive.