PTC stands for Positive Temperature Coefficient and is simply another name for a thermistor.
A PTC device will have a nominal resistance, for instance 1kohm, that it is normally at. If it gets hot then its resistance increases.
PTC’s are often used to protect inputs for instance in multimeters but not they are no fast-acting. They have a thermal mass and take time to heat up and increase their resistance. They are great for protecting against long term overload events, but not for fast-acting overloads – for those a MOV Metal Oxide Varistor is often used (often in combination with a PTC)
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