What does the ISIS flag mean?

Recently we have all been seeing the shocking and disturbing images of the fighters of ISIS or whatever they call themselves now, on our TV screens. They use their flag quite a lot. But what is their flag and what does it mean? Black flags are common symbols of militant jihad terrorist and Islamic extremists, however the symbols of the flags themselves are not at all militant until used in that way. In order to undermine these groups and what they stand for we should examine the symbols they have stolen and twisted for their own evil advantage.
This interesting video explains the meanings of the symbols used by the terrorists: