What to Write
Most college essay prompts, including the common app’s prompts, ask in some way why this college should accept you. Your essay’s purpose is to move an admissions officer from “I don’t know this person” to “This person should come to this college.” Everything you say should work to that end, even if it’s by way of talking about overcoming obstacles or questioning a belief or whatever else the prompt may specify.
Use the words that state your message best. Fancy words exact a cost in your reader’s attention, but may be worth using if no plain words do; unnecessarily fancy words could annoy a reader and give the impression that you are showing off.