IQ (intellectual quotient) is a score measuring intelligence.
It is measured with multiple subtests, in order to closely match General intelligence. They typically include matrix reasoning, lexical span, digit span memorization, number sequence deduction, and processing speed.
The most common and standard test batteries are WAIS (Wechsler adult intelligence scale) and WSC (Wechsler scale for children).
It is standardized to have a mean of 100 and a standard deviation of 15.
Correlations and predictive power
It has predictive power over many life outcomes: IQ in childhood is strongly correlated with academic success1, and moderately correlated with income2.
It also predicts lifespan, even to some extent within people with IQ higher than 130.
Some of this predictive power stems from the fact that IQ is partly environmental and caused by living conditions determined by social class, but some of this predictive power remains when controlling for socio-economic status and parent income.