What is the difference between a patent agent and a patent attorney?

A patent agent is an individual who has been authorized to practice patent law before the United States Patent and Trademark Office.  To become a patent agent, you will need to have passed the Patent Bar Exam (and to take the Patent Bar Exam, you will need to first have earned a bachelor's degree in engineering or one of the hard sciences).  

A patent attorney is an individual who accomplished all of the above, and in addition, is licensed to practice law in at least one state.  Therefore, a patent attorney has attended an accredited law school and has passed the bar examination of at least one state.