PTC stands for Positive Temperature Coefficient. This refers to the silicon guts inside the relay whose resistance increases as its temperature rises. When the compressor first starts up, the resistance of the PTC stuff is very low and it lets current flow into the start winding of the compressor. As the current flows thru the PTC, it heats it up and also raises its resistance to the point that the start winding is effectively taken out of the circuit, which is what you want to happen as soon as the compressor gets going.
Back in the good ol' days, compressor start relays were the mechanical type. This was a metal plunger inside a tube with a heavy gauge copper coil wrapped around it. As current flows thru the copper coil, it creates a magnetic field that moved the metal slug up and took the compressor start winding out of the circuit thata way.
Grand Master Funk kdog has put together some examples showing common PTC compressor relays and a good ol' mechanical, non-PTC type relay; click the links above the pics for a larger view or to order the part:
Some common examples:
This is what a NON-PTC starter looks like:
Source: Whirlpool acronym