The tests measure aptitudes in categories differentiated by notable psychologists. I'm way too tired to go into that right now though....
Intelligence scores are an overall calculation but there are underlying scores for each category to show the strengths and weaknesses of someone's mind. The real problem faced in the credibility of aptitude tests is whether (1) whether the test is [culturally] biased, (2) whether the material given really does effectively serve as a measure of someone's aptitude in that category and (3) whether the measurements of the responses given can be quantified in a manner that reflects intelligence properly. In other words, the subjective approach of a corner in the psychological community decides what the best measure of intelligence is based on their experiences and education, rather than just any old guy. Nonetheless this doesn't mean they're right, but it does lend a bit more weight to IQ tests rather than just saying they're no good without really specifying why...