When applying to most colleges in the United States, you will have to write essays for each college based on a specific prompt that they give you.
Common college essays prompts are:
Why us - colleges ask you why you want to attend their college
Why do you want to attend this university - colleges ask you what you would contribute to their school, sometimes included in “Why us” essay
Personal essay - all students write this: you usually talk about a challenge you faced, or some important event or characteristic
Why this major - talk about why you want to get a degree in whatever subject you choose