study.com

Great! You're finally off to college and ready to begin the next step in your life. While in college one thing that you're gonna get used to is writing essays. In order to compose a proper essay, you have to have a good understanding of the rhetorical concepts needed for the essay. There are plenty of concepts that can be used, but the two most important are audience and purpose.

Having an understanding of both of these will make it easier for the author to know what to write about and to whom he/she is writing to. Also, having a clear idea of what it is you're trying to accomplish by composing your essay will make it a lot easier to decide what information you want. Before thinking about purpose it's important to already have your audience in mind. This is important because certain audiences have specific interests, and if you're writing to the wrong audience the purpose of your essay can be meaningless. Once you have your audience, it's time to decide whether your paper is going to be expository, narrative, or descriptive. Having this knowledge will benefit you through out your entire college experience and make writing essays a whole lot easier.  

This is an excerpt from an informal project called concept tracker. Gives a better understanding of rhetorical concepts such as purpose.