The main essay that is read by all colleges you apply to through the Common Application is your Common App essay, so be sure to make this very strong and broadly applicable. The Common App essay is typically something that talks about your personal experiences, something that has happened in the past that has influenced you, for example. Earlier this year, The Admissions Blog gave some advice about where to start with the new Common App essay.
The college essay, on the other hand, is your chance to personalize your statement of interest to a particular college. The college essay is meant for the college to understand more about you, and specifically how you would fit into their school. The college essay is also a way for the college to ask you a specific question they want to hear about from you. So, although the common app essay is the broader one that everyone reads, it's still important to write a strong college-specific essay.