Howard Gardner , an American developmental psychologist, broken down intelligence into 9 categories in a book called Frames of Mind: The Theory of Multiple Intelligences . However, IQ tests typically measure general intelligence in only one to four basic categories of ability:
IQ tests are all standardized, meaning the scores are representative of a certain population. This representation follows a normal distribution, or a 'bell curve', where most people score near the middle. Using a standardization process allows those who define high, medium, and low score definitions to identify norms and standards to use when comparing people's scores. Typically, 100 is used as the median score.
In short, this means that if you exceed 100, you have an above-average intelligence. A lower score indicates that, in a certain extent, you are less intelligent than the average.
The majority of test creators agree that a score of the median plus or minus 10 to 15 (85 to 115) is indicative of ‘average' intelligence. Any score below a 70 is considered to be a low IQ, while a score above 140 is considered to representative of highly exceptional intelligence.
Modern tests consider age (and appropriately so) when determining a score, meaning a person is graded relative to the population of his/her developmental level. Children, for example, especially benefit from this.