In World of Warcraft, I know there are instances and raids. I'm pretty sure dungeons refer to raids and instances. But I don't understand how heroics work. All I know is that raids are instances that allow more than 6 people. But I don't understand which raids or instances can be heroic. But then I also heard that some raids can be done with 5 people, and some Classic instances can be done in heroic mode with 6 or more people. Can someone please explain all this?