Gotta agree with "LexDiamondz" on this one (great answer, by the way). The facts first...
The aim of an IQ test is to measure a person's intelligence, which is supposedly an indication of that person's potential. The average IQ, (measured by mental age/chronological age x 100) is 100, though most people fall between 85 and 115.
Personally, I don't necessarily agree that these tests indicate intelligence, mainly because A) it's hard to develop a test that measures actual intelligence without introducing some kind of cultural bias, and B) there are different kinds of intelligence. Let me explain...
IQ tests rely largely upon symbolic logic as a means to scoring, which is not inherently synonymous with intelligence. For example, it's possible that someone could have a wealth of emotional intelligence while being simultaneously unable to understand the significance of sequentially arranged shapes. Additionally, someone who can't read would be at a huge disadvantage on an IQ test, though being illiterate doesn't mean that person isn't intelligent. Also there are other factors to consider, such as income level, education level, nutrition level, environment, culture, etc., that all contribute to both how well or how poorly a person is able to learn and retain knowledge and the opportunities and tools available to them. Those same factors also contribute to what specific kinds of knowledge a person acquires (e.g., "street smarts" vs. "book smarts", etc.)
So in short, it seems that IQ tests in general are basically a measurement of how well a person has learned (or adapted to) a specific skill set valued by those who created the tests.
Just my two cents.