Portal Keys are fairly small, golden hued keys. There are several different keys that appear throughout the series, all of which came from Discord, and for each owner the handle of said keys would have their heads engraved into the gold handle, shown smiling.
Despite their mundane appearances, these keys hold a sort of magical property that connects them to their owners. When they're pushed into a door or other surface, they're able to open a portal to another dimension, though they're only shown to be able to go from Earth to Equus and nowhere else, as shown when Peridot tried to use said keys to get to Homeworld, only going back and forth between said worlds. Exact locations on where they open to are consciously, or sometimes subconsciously decided by the owner of the key. The keys can only be used properly by their owners, and anyone else who tries a key that's not theirs won't be able to activate them, shown when Amethyst tried to use Steven Universe's key, with no success.
There's even a sense of sentience evident in these keys as they react to their owners in emotion, capable of laughing, and actually can move around on their own accord evident with Steven and Connie's keys.
The first key that appears was shortly after the Mane Six were able to return to their world, and Discord dropping off Steven's key first. It doesn't start to activate until Steven tries to show Lapis Lazuli to the others, which by then it jumped at Garnet, and got Steven to follow it outside. After Lapis got released, it opened a portal to the Castle of the Two Sisters, cutting off the portal before the other Crystal Gems could get them.
Three more keys, each one for Garnet, Pearl, and Amethyst were given to them after "Daring Do, and the Aqua Gem".
Steven was able to use the key to go to Sweet Apple Acres alongside Lapis and Amethyst, and Connie's key was given when the Spider Monster attacked the Crystal Temple, and she was able to use it to go to Equestria and to wherever Steven was. This same key comes to life later and makes a portal for Connie after she was punished by her parents. This is the first example of a sub-conscious use of the key.