What is the difference between category 5 and category 3 telephone wire?

Originally, there was only the true telephone RJ45. It is one of the many registered jacks, like RJ11, a standard from which it gets the "RJ" in its name. As a registered jack, true telephone RJ45 specifies a physical connector, and also the wiring of it. The true telephone RJ45 uses a special, keyed 8P2C modular connector, with Pins 5 and 4 wired for tip and ring of a single telephone line and Pins 7 and 8 connected to a programming resistor. It is meant to be used with a high speed modem, and is obsolete today.