From wikipedia on intelligence:
"Intelligence is an umbrella term describing a property of the mind including related abilities, such as the capacities for abstract thought, understanding, communication, reasoning, learning, learning from past experiences, planning, and problem solving."
(
http://en.wikipedia.org/wiki/Intelligence)
Seems to me it can be summed up nicely, and perhaps a touch poetically as "the process of determining one's reality".
Intelligence is no safeguard against stupidity. But reality, for any single individual, is determined by how perception is understood by this individual.
So, we all live in the same world. But if Joe, who has a degree in rocketscience, and Bill who only ever shined shoes for a living, both were to look out of the same window, they wouldn't see the same things. Their perception is affected by their intelligence, and their intelligence is molded and shaped according to how it is used.
Having realized that, the next step for anyone interested in understanding and mastering their intellect, would be to examine how they use it. It is the zen approach, and I honestly believe it can drive sane men to the very edge of their rationality. But that's a good thing, as long as you don't fall off..