Scroll down a bit in this article. There's a list of what each of the available keys are required to provide. A "key" in this context is basically a notch in a certain location, which then defines the meaning of the various pins of the connector. Some devices have multiple keys, as some of the specifications have a common subset. Like key A+E is common, because E provides almost everything that A does, so a device that only requires the common interfaces can work in both. Cars that rely on one of the exclusive interfaces will have the specific key of course. This A+E communication is often used for WiFi cards.

Sockets always only have one key though, for obvious reasons.

Edit: correction/clarification