The Law School Admission Test or LSAT is a standardized test that must be taken by all persons seeking admission into law school. This test is required by all 197 law schools that are members of the Law School Admission Council. Every person who wants to attend law school must take this exam which provides the same types of questions, roughly the same number of questions, under similar testing conditions across the board. This is critical since there is such a diversity of people applying to law school who attended different types of colleges and universities and who earned degrees in various fields of study. With the LSAT, that one score will allow schools to compare different people against each other.
Many schools weigh the LSAT score most heavily.
It depends on the school. One thing that is certain is that you MUST take preparation for this exam very seriously. You cannot afford to wing it. Doing well on the LSAT will greatly increase your chances of getting into a law school of your choice. Doing poorly will no doubt limit your options.