What means MutVar# ?

Hi, by trying to understand how "writeIORef" works, I looked at data STRef s a = STRef (MutVar# s a) what is MutVar# ? (especially zhe "#" ) ? I've readed tha writeIORef (and readIoRef etc) is used to write directly to memory places fo implementing variables, as in example in HOpenGL used. Is this also Haskell standard ? Thanks for help Hans