Just about every college or university application these days requires the student to write an essay. The whole point of these essays is for the student to make himself or herself come alive - be an individual - in the mind of whoever reads it. One thing to avoid is repeating information the admissions committee already has - grades on the transcript, test scores, application information, etc.
The first thing to be careful about is to follow the specific directions on each application. Some schools ask applicants to write a biographical sketch (pretty broad topic!), while others ask specific questions and expect responses that address those questions. Follow those directions explicitly! Anything off-topic indicates to the admissions committee for that school that the student cannot (or will not) follow directions.
There are many places online to get help writing that college app essay. One of the best places, of course, is at The College Board's website: http://www.collegeboard.com/. Just click on For Students (at the left) and then, a little right of center on the next screen, click on College Essays in the Apply to College section. The Dos and Don'ts on their College Essay Writing Tips webpage are excellent.
Other places to get help can be found by going to http://www.google.com/ (or any other search engine) and searching for college application essay or college admission essay. Just be wary of any that promise to write it for you or that want to charge you for entering their site, "mentoring" you, or proofing/editing your essay.
And then there's this from the Washington Post:
http://www.washingtonpost.com/wp-dyn/content/article/2007/10/16/AR2007101600543.html?hpid=news-col-blog
What an excellent collection of don'ts from this writer!
If anyone out there is writing a college app essay and wants input or feedback, just let me know. Post your questions in comments.