What are the uses of Current and potential transformer?

Both Current and Potential Transformer are instrument Transformers which are used to measure very high current and voltage respectivly.

Current Transformer is a Step up transformer and used to measure high current, its secondry is connected to ammeter of very small capacity (usally 5 A) but its Scale is calibrated according to actul values.
Potential Transformer is a step down tansformer and its secondry is connected to voltmeter of small range but again the scale is calibrated for actul values.
So as a whole both Current and Potential Transformers are used to measure high values of Current and Voltage respectivly.