Many college applications require a personal statement, essay or short answer.  You can typically determine if an essay is required by visiting the admissions tab on a a college's website and finding the list of "required documents" needed to apply.  Some colleges will give you the freedom to write on a topic of your choice, while otehrs will supply a specific prompt or question.  

The Common Application historically has six prompts from which to choose and some colleges will require an additional supplemental essay.  

The college essay is an important part of your application.  It is your chance to tell the admissions committee who you are and is a way to express yourself beyond the information contained in the rest of your application.  You want to make sure your essays stands out from your peers.  Below are some resources available to assist students with the essay writing process.  Remeber - this is not something you can complete in one day.  It will take time, but the work you put into your essay will show and be a better reflection of you who you are beyond your transcript and application.