The spec of the interconnection really. It goes back to the analogue days. Has rgb and composite and stereo sound both ways and a pin with voltage on it to do switching of inputs. However some scarts are not complete. I've come across them with just composite, no RGB at all and no switching pin. Also when the different aspect ratios started to come in other pins in the Scart started to be used for that switching as well. I guess you can look on it as the analogue version of hdmi but without the anti piracy built in to the latter system, and no HD.
Brian