Well by the looks of the other explanations, you still know basically nothing about them (no hostility :) )
An Oc is an Original Character. A character that you made. It does not have to be your art (though making your own drawings is strongly encouraged!) It Just has to be your character. It cannot be the exactly the same as another Oc intentionally. Also, please ASK (looking at you, Photon bee) when you draw a species by another creator. While Photon does not follow that rule in particular, you should. Take my species, the Pinlo. It is not illegal to use it, I just prefer you didn't and I will ask you to delete it. But you don't know a lot about them, so then but doubt you will try to take one.
Ocs can be used in a variety of ways. To personafy yourself as a different thing. To write a story. To roleplay as said Ocs. The possibilities are endless. Do not use other people's Ocs as your own, please. So not judge any characters negatively please.
This is the basics.