Ivy League Colleges including Harvard, Princeton, Yale...
their formal names all include "University".
Harvard College is the undergrad portion of Harvard University.
Surprisingly, wiki has a damn good writeup on the distinction between College and University in American English. We completely bastardized the whole system here. Excellent.
But in short, wiki sums it all up excellently: the proper term college in the U.S. is almost always for a school that is either a "junior college" that is focused on 2-year degrees, a vocational approach if you will, or for schools that focus on undergrad studies and have little or no graduate-level faculty/instruction.
The "colleges" within a university, are simply borrowing the name, and have no part in the college/university discussion.
And some institutions in the U.S. have kept the old College name just for the hell of it, which further bastardizes the naming scheme. Others keep the College name to show they focus more on undergrad than anything else. Universities, on the other hand, are almost always research focused and integrate the graduate studies into just about everything.