In general, percentile ranking is a measure of the percentage of people who score at or below a given level. So, at the 50th percentile, 50% score at or below that level. At the 75th percentile, 75% score at or below that level. And so on.

Percentile rankings are specific to one test and are used as the measure of how well any given individual did on that test.

GMAC does not publicize HOW its algorithm works (that is, how they calculate our scores and how they combine the math and verbal sub-scores to reach an overall score and percentile ranking).

I'm not quite sure what you're asking in the second half of your question - are you asking whether the GMAT has test questions about the concept of percentile itself? It's not very common. You should definitely expect to see percentage questions - but you may not see anything relating to percentile.

Got it. Percentile questions are rare. Percentage questions are very common.