This is an important part of your application. Make sure you jump off the page!
Tell them something about yourself they won’t see in your transcript. Tell a story and keep them interested. They’re reading lots of essays, so make yours memorable. Avoid discussing your friends (they might want to accept them), the death of a grandparent (it happens to everyone), or drugs and depression; do tell them about things you’ve overcome. Tell them about fears you’ve conquered, or that moment in your life when everything changed, when you looked into the mirror and got a glance at something (someone) pretty significant.
Unless you’re telling a really fantastic story, try to keep your essay on a single page (single-spaced). Make sure your name is on EVERYTHING you submit. When you think you have your essay in good order, take it to your parents, your teachers and your Aunt Millie who works in publishing and get their opinions; then implement the ones you want. It's your essay.
Don’t think about what THEY want to hear, think about what YOU want to tell THEM. Admissions officers read plenty of essays about the charms of their university, the personal commitments involved in being a doctor or adventures at summer camps. Bring something new to the table! Don't include information that is found elsewhere in your application. Your essay will end up sounding like an autobiography, a travelogue or a laundry list. And finally, don’t forget to proofread! Typos and spelling or grammatical errors can be interpreted as carelessness or simply bad writing. Don’t rely on your computer’s spell check.
Be positive! And be yourself!