What is the difference between a schedule I and schedule II controlled substance?

A schedule I drug is one for which a prescription cannot be written, such as heroin, LSD, crack, etc.

A schedule II drug is one which doctors can write prescriptons for, but have a high potential for addiction and abuse, such as Demerol and Morphine. Refills on these drugs are not allowed; you must have a new prescription written each time you get them.