Most university applications require a student to write at least one essay so that the admissions committee can get a better idea of the student. Whereas grades & test scores provide information, an essay can personalize an application. It can show the university who you are and what makes you unique.
According to The College Board, The National Association for College Admission Counseling’s 2011 State of College Admission report found that while grades, strength of curriculum and admission test scores are the top factors in the college admission decision, a majority of colleges and universities believe the essay to be of considerable or moderate importance in determining which academically qualified students they would choose.
Admissions Offices will give a student various essay prompts and will typically ask a student to choose among them. The Common Application typically announces the annual prompts.
The Princeton Review gives some helpful advice when it comes to writing the college application essay.
USA Today also gives some good feedback when it comes to crafting a strong essay.
The best overall advice is to PROOFREAD, PROOFREAD & PROOFREAD. Most universities also encourage you to have other students & teachers look over your essay!