A mutation is a permanent change in the DNA sequence of a gene. This can be beneficial if the change gives a new function to or improves the function of that gene.
The above is a definition. But one must really define "beneficial". Some regard it as beneficial if it helps the specific individual who has it. Others would think it beneficial if it produced some survival advantage that insured more descendents for that individual.
It is the difference between a mutation that allowed for greater athletic ability, but a decreased desire for offspring, versus a more moderate athletic enhancement, but a greater desire for offspring.
The small percentage of ways to improve an organism, versus the near infinite ways of harming the organism, mean that most mutations are not going to be beneficial. At best, they will be "inconsequential" - such as a new shade of eye color, or a mole on a section of your skin.
The populist notion of "powers" that can come from mutations is wildly inaccurate. Even assuming a minor power like the ability to see infra red radiation would take thousands upon thousands of mutations over vast amounts of time. A mutation for blindness is far more likely.
It should also be noted that the traditional model of evolutionary theory no longer applies to man. We don't allow changes in our environment, and without such changes, there is no need for one trait more than another to predominate. After all, it is irrelevent that a mutation might allow for greater speed in running, when everyone drives a car.
