What is its Purpose?
Your graduate school application provides the admissions committee with a great deal of information about you that cannot be found elsewhere in your graduate application. The other parts of your graduate school application tell the admissions committee about your grades (i.e., transcript), your academic promise (i.e., GRE scores), and what your professors think of you (i.e., recommendation letters). Despite all of this information, the admissions committee does not learn much about you as an individual. What are your goals? Why are you applying to graduate school?
With so many applicants and so few slots it's critical that graduate admissions committees learn as much as possible about applicants so as to ensure that they choose students who best fit their program and are most likely to succeed and complete a graduate degree. Your admissions essay explains who you are, your goals, and the ways in which you match the graduate program to which you are applying.
What Do I Write About?
Graduate applications often ask that applicants write in response to specific statements and prompts. Most prompts ask applicants to comment on how their backgrounds have shaped their goals, describe an influential person or experience, or discuss their ultimate career goals. Some graduate programs request that applicants write a more generic autobiographical statement, most often referred to as a personal statement.
What is a Personal Statement?