Essay(s) are written for US schools. The Personal Statement is written for UK schools.

JUNIORS: Check out lists of essay topics from previous years (here and here and here and here) and write some practice essays before your senior year starts. Check out what UCAS wants in a personal statement and practice writing one. Answering these questions assists the admissions committee AND forces you to ask yourself questions you may never have considered. You will learn more about yourself and maybe about the kind of school you want to apply to, plus you can probably use bits and pieces (or whole essays?) in the real essays you write before Christmas senior year. 

This 2 minute video from the William and Mary admissions committee is from 2008, but if the narrator had said "Facebook" instead of "Myspace", you'd never know it wasn't made for this year. Everything else is still completely and utterly true.

Most schools limit the number of words. Stay under that limit. The main reasons for setting a limit: 
  •  Admissions officers have limited time and receive MANY (hundreds, thousands - no kidding) essays. 
  •  Schools want to know whether you can write concisely and well. 
  •  Schools want to know whether you can follow directions. 
If there is no word limit, don't write more than 500-600 words.

And think about your words: don't just dump out what's inside your head.

Schools expect you to ask for advice on your essays and personal statement, but the finished product must be yours. (Admissions officers read so many essays that they are very clear on the difference between the voice of a high school senior and a 47 year old mother or teacher. You won't fool them.) Ask people who know you to read what you wrote. Then ask, "What works? What doesn't? What sounds like me? What doesn't? Does this help you understand me? Is the writing clear? Does what I wrote answer the question?" Then take that feedback and revise, then revise again, then keep revising until your college counselor tells you that you have a really good essay or personal statement. (NOTE: this cannot be done if you start a week before the finished product is due....)