Most if not all graduate schools require you to write a statement of purpose essay. The specific question may differ from school to school, but generally, schools want to know:
According to Robin Mamlet and Christine Vandervelde (College Admission, Three Rivers Press, New York) you can follow a 10-step day-2 week strategy toward a great essay.
Many colleges/universities ask Why Did you Choose to Apply to College X? or What aspects of the curriculum or undergraduate experience prompted your application?
Purdue University has an excellent Online Writing Lab. Look it up!