About the GMAT
The GMAT is a standardized test. Each time it is offered, the test has, as close as possible, the same level of difficulty as every previous test. Maintaining this consistency is very difficult--hence the experimental questions (questions that are not scored).
The effectiveness of each question must be assessed before it can be used on the GMAT. A problem that one person finds easy another person may find hard, and vice versa.
The experimental questions measure the relative difficulty of potential questions; if responses to a question do not perform to strict specifications, the question is rejected.
About one quarter of the questions on the GMAT are experimental. The experimental questions can be standard math, data sufficiency, reading comprehension, arguments, or sentence correction. You won't know which questions are experimental.