A CT is a Current Transformer. Its purpose is to transmit a low-level current from a high one. For example, if a circuit is flowing 3000 amps (Yes, sounds like a lot, but there are tons of them even higher than that), no meter is made that can handle that much current directly. So a CT is used.
All CTs have ratios. Most of them have an output of 5 amps, and the input will vary as needed. In the above example, A CT with a ratio of 3000:5 would result in exactly 5 amps to the meter when 3000 amps is actually flowing in the circuit. If there's 1500 amps in the circuit, then there'd be 2.5 amps to the meter.
There are different classes of CTs, mostly having to do with accuracy.
Odd as it may sound, all CTs operate at a short circuit. If a CT is operated on an open circuit, VERY high voltage will be seen at its terminals, and it will be destroyed instantly.
Pts and Vts are the same thing. Potential transformers and Voltage transformers. These are very much like any other transformer, as they change voltage, but a PT is far more accurate.
One use for PTs is in metering high voltages. No meters are made that can read 14,400 volts directly, but if a PT is used, 14,400 volts becomes 120 volts. Most PTs have only one number stamped on them, the secondary is assumed to be 120 unless otherwise noted. So the above example would be labelled 14400. If 7200 volts were applied, the output would be 60 volts,
As with CTs, PTs come in a variety of different classes, based mostly on accuracy.
The 120 volt side of a PT has very little current, It is not used to power anything other than signals at meters and other instruments. Never to power anything that needs more than a few milliamps of current. If any kind of actual load is placed on a PT, it will no longer be accurate.
This is just a brief overview, there are tons of other applications for both CTs and PTs.