For a more general, encyclopedic overview:
The LSAT, or Law School Aptitude Test, is a standardized test administered by the Law School Admissions Council, a private, non-profit organization. Its administration is mandatory for anyone who wishes to study at an accredited American law school and attain a J.D. (juris doctor), which in turn is a requirement for anyone who wishes admittance to the bar and the right to practice law in the United States. Candidates usually take the test during either the junior or senior year of their undergraduate education but may sit for it at any time before they apply to law school.
A law school applicant"s LSAT score and, to a lesser extent, his grade point average, are the two numerical factors that determine his acceptance to law school. The test has no applicability or relevance aside from this specific purpose and has no further bearing on the student after he first begins his enrollment.
The LSAT is designed to measure a battery of different skills, each, ostensibly, with a degree of applicability to the study of the law. The key word there is 'skills,' not 'knowledge' – one need not prove a thorough understanding of the American justice system, legalistic jargon, or police procedure in order to do well on the test, but show that a strong foundation for learning to work with such concepts is present. The four separate categories are:
Logical Reasoning – You get to read a statement or short passage and then answer a question regarding its implications to the best of your common sense reasoning abilities.
Reading Comprehension – You get to read a passage on some fairly dry or academic topic, about 70 – 100 lines in length, and answer some questions which test your ability to absorb and analyze the material therein.
Analytical Reasoning – You"re given a set of logic matrices consisting of two or more variables and must order them based on an incomplete set of information (which usually varies from problem to problem) with a list of rules and conditions to which you must invariably adhere.
Argumentative Writing – You receive a hypothetical situation which outlines two goals that a party wishes to achieve, and two possible courses of action (usually business models) to pursue. You must evaluate, choose, and advocate one of these options in a grammatically correct and logically cogent essay of about a page in length. Do not skip lines.
The first three categories involve use of a multiple choice answer sheet and last 35 minutes each; the latter is a free-writing exercise half an hour in length. Students can work on each section only in the allotted time period. The multiple choice part of the test consists of two sections of logical reasoning, one of reading comprehension, another of analytical reasoning, and a fifth, non-graded 'test' section included for research purposes, which may be any one of the three multiple-choice categories, except for the fact that it is not graded. It is also unrecognizable from a normal, graded section.
In all, there are five sections plus the writing exercise, comprising about four and a half to five hours of total test time (allowing for registration and other procedures). A break of ten minutes comes after the completion of the third section, but aside from that there is no reprieve, so students taking the LSAT need to utilize their reserves of physical and mental stamina considerably to function at peak efficiency throughout the test's administration.
Currently, LSAC scores the test by converting the 'raw score' -- the number of correct answers out of the number of total problems – to an adjusted score from 120 (lowest) to 180 (highest). Each problem counts as one point toward the raw score. However, with adjusted scores, LSAC weighs both the number of problems (which varies from test to test) and each test"s relative difficulty, assigning a handicap curve of a few points to its more strenuous administrations. 150 is considered to be at about the 50th percentile of overall test takers and therefore a mediocre (but not terrible) score; 163 is at the 90th percentile and considered superior to the standards of most law schools; and 172 is at the 99th (which, barring other considerations, assures a good shot at admission practically anywhere).
Speaking of admissions, you can find valuable information on every accredited school in the country by checking out a copy of the Official Guide to U.S. Law Schools. Many, but not all of the school listings will each have a table that will contain statistics of a representative pool of applicants. You can cross-reference G.P.A. within a certain rank and an LSAT percentile and come up with the number of the applicants who applied with such scores to the school as well as the number of among them that the school accepted; from that, you're able to calculate a rough percentage of acceptance and thus gauge your own probability of getting in.