The MCAT is the standardized exam associated with medical school admissions. Nearly all allopathic and osteopathic medical schools require the MCAT as a prerequisite for admission and, furthermore, use the MCAT as an important selection factor for admission. The MCAT is an assessment of critical thinking skills, problem solving skills, and competence within prerequisite subjects of the study of medicine.
If you plan to take the MCAT in 2014, learn about the current MCAT at:
https://www.aamc.org/students/applying/mcat/about/
Beginning in the Spring of 2015, the MCAT will be a different exam than its previous two editions, the first of which was implemented in 1991 and its amendment to remove the Writing Section in 2013. The most significant change to the MCAT will be its inclusion of questions from the social and behavioral sciences (psychology, sociology, behavior science, etc.), which was excluded from the previous editions, focusing only on biological sciences, physical sciences, and verbal reasoning and comprehension.
To read the full description of changes and the new MCAT, click the links below. The first is the AAMC's "Preview Guide for the MCAT 2015 Exam (Second Edition) and has a comprehensive description of what you need to know, and the second is the general website for the MCAT 2015 Exam.
https://www.aamc.org/students/download/266006/data/2015previewguide.pdf