What is the difference betwen current transformer and potential transformer?

A current transformer is a donut (toroidal) shaped transformer. You use it to measure current (expressed in amperes). You run your wire to be measured through the transformer and it produces a variable output based on the current.

A potential transformer is basically a high ratio transformer that is used to measure voltage on high-voltage lines. For example: They will use a 10:1 ratio potential transformer to convert 4160v to 416v. Then they can measure this lower voltage with a meter. The meter then applies a user programmed multiplier (10) to this number to calculate the voltage.