To answer the OP's question, no there is not a consistently-accepted, widely-known difference (as this thread demonstrates), and people often use the words basically interchangeably (although they will say "college" much more frequently). Some schools do use the system where the entire school is referred to as a university, and it contains multiple colleges (college of engineering, etc.).
I don't know why we use the terms this way, but we do.
I don't know why we use the terms this way, but we do.