There is a difference between a college (diploma) and a university (bachelor, masters, PhD degree). A college diploma is a degree received from a two-year institution of higher learning while a bachelors degree is a degree earned from a four-year institution of higher learning. The difference is not limited to the time spent at the institution but can also include the salary differences and the level of a job position. This is not to say that one with a bachelors degree will earn more than one with a college diploma. There are many graduates from a college who are successful (success is not measured by wealth but what one accomplishes in life and the happiness of the individual).
The above answer applies more to Canada than to the US. In normal US usage, when someone says "college diploma" they mean a Bachelors Degree from a college or univesity. Technicall a "college diploma" could be any degree (Associate's, Masters or Phd.) but that's not how it's used.
A diploma is the piece of paper that says you have a degree.