IQ tests measure.... the ability of solving IQ tests.
Now, whether this ability is equivalent to what we call intelligence, is a hard question to answer.
The skills needed to solve IQ tests are predominantly based on math. They may be important, but is 'intelligence' not broader than that? A historian which proposes a new way to look at some historic event may be called "brilliant", but being a dunce in math, would be do good on an IQ test?
For sure, it is an accomplishment if you do well on IQ tests, but I do think that a very narrow set of skills is measured by it.
These tests reward people who have an interest in math, puzzles, games like chess and go, programming, etc. These are specific interests and hobbies, and if someone has different interests, it does not mean he is stupider. However, an IQ test will reward the person who likes to solve math problems every evening, while someone who prefers to read literature, go to the pub or to play computer games, will have a harder time. Does this mean this person is less intelligent?
Attitude is very important. Some people simply don't have the patience to solve these problems. Maybe they haven't been put through school, where they were trained to sit down and do problems. Does this mean they're stupid? Also, it has been shown that people generally do better on their second try of an IQ test, than on their first one.
Basically, IQ tests are important, but it is a bit of an stretch to say that what they measure is 'intelligence', at least in the everyday meaning of this word.