What is the difference between Crude Oil and Petroleum?

Here I just extracted answers from U.S. Energy Information Adminstration:

Crude oil is a mixture of hydrocarbons that exists as a liquid in natural underground reservoirs and remains liquid when brought to the surface. Petroleum products are produced from the processing of crude oil at petroleum refineries and the extraction of liquid hydrocarbons at natural gas processing plants. Petroleum is the broad category that includes both crude oil and petroleum products. The terms "oil" and "petroleum" are sometimes used interchangeably.

Here is the link of the original information
http://www.eia.gov/tools/faqs/faq.cfm?id=40&t=6