Brainfuck is an eight-instruction Turing-complete programming language—one of those languages like INTERCAL, Unlambda, and reMorse that are designed to be essentially unusable.
Here's Hello World, as small as I can make it (156 bytes):
++++++++[>+++++++++<-]>.<++++[>+++++++<-]>+.+++++++..+++.<++++++++[>----------<-]>+.<+++++[>++++++++++<-]>+++++.<++++[>++++++<-]>.+++.------.<+++[>---<-]>+.