In Final Fantasy 7..crisis core...?

Yeah... so i never played crisis core, but ive watched advent children, and my brother tried to explain it to me. I think i got the main parts of it down...but at the end of crisis core, theres a mini clip of aerith like in this weird place, and i thought she died, because in advent children she was up in heaven... so...what basically, what happened between crisis core and advent children?