IQ tests are scored in a relative way. There is no absolute measure of intelligence; instead, the results tell the test taker where she falls in relation to peers her age. The score itself comes from the test taker's "mental age" as discerned by her score divided by her chronological age.
IQ tests were originally developed in order to find learning impairments in children. Today these tests have been honed and revised and are still used for this purpose. For educators, IQ tests can be an important way to discover children with learning disabilities and special education needs.
There is a history of controversy surrounding IQ and what it actually measures. The tests have been criticized for being biased toward culture, race and class, and questions remain as to the accuracy of the measurements.