Prime95 is basically a Mersenne prime number discovery program. It's a great example of Distributed Computing, but it's most loved by overclockers for its powerful system stress-testing and benchmarking abilities. It can torture-test your CPU to produce its maximum heat or spit out any errors due to excessive overclocking. Also, using the blend test, it can consume all your physical memory and run error-checking tests on your modules to ensure their quality. However, right now, we're mostly interested in it for its benchmarking capabilities.
Here are the times for the Intel Celeron D 341 and D 346, Intel Pentium P4 530, and AMD 3000+ (less is better).
There is a huge difference between the Intel and AMD processors while there is not much between the P4 and Celerons.