The total GMAT score ranges from 200 to 800 and measures performance on the quantitative and verbal sections together (performance on the AWA and IR sections do not count toward the total score, those sections are scored separately). Scores are given in increments of 10 (e.g. 540, 550, 560, 570, etc.). From the most recent data released by GMAC, the average GMAT score of all test takers is about 540.
The score distribution conforms to a bell curve with a standard deviation of approximately 100 points, meaning that 68% of examinees score between 440 and 640. More precisely, the mean score is 545.6 with a standard deviation of 121.07 points.
The final score is not based solely on the last question the examinee answers (i.e. the level of difficulty of questions reached through the computer adaptive presentation of questions). The algorithm used to build a score is more complicated than that. The examinee can make a mistake and answer incorrectly and the computer will recognize that item as an anomaly. If the examinee misses the first question his score will not necessarily fall in the bottom half of the range.
After previewing his/her unofficial GMAT score, a GMAT test taker has two minutes to decide whether to keep or cancel the GMAT score. If the score is cancelled any future score report will still note that the test taker sat for the GMAT on a certain date. The score will be noted as a "C" and will remain on the score report for 5 years. A cancelled score can be retrieved within 60 days for a fee of $100. After 60 days a cancelled score is not retrievable.