They measure more than one area of intelligence. There is an attempt to make them interesting so that the subject puts forth his or her best effort. They are normed and standardized on a sample population. There are different theories of intelligence. Some tests were designed to measure crystalized intelligence and fluid intelligence.
Crystalized intelligence is that which is learned.
Fluid intelligence would be the ability to solve a problem which one has never encountered before. Other tests measure verbal abilities as compared to performace or visual abilities. Still other tests measure sequential processing as compared to simultaneous processing or left brain functions as compared to right brain functions. All intelligence tests require extensive training to administer and interpret. Only qualified professionals should dertermine when an intelligence assessment is needed, administer intelligence tests, and interpret them.