Brainfuck is a minimalistic programming language and a computation model. It is very easy to learn the language, but it is very hard to do coding even for simple computation tasks.
The following introduction to brainfuck is borrowed from here.
Brainfuck is the ungodly creation of Urban Müller, whose goal was apparently to create a Turing-complete language for which he could write the smallest compiler ever, for the Amiga OS 2.0. His compiler was 240 bytes in size. (Though he improved upon this later -- he informed me at one point that he had managed to bring it under 200 bytes.)
I originally started playing around with Brainfuck because of my own interest in writing very small programs for x86 Linux. I also used it as a vehicle for writing a program that created ELF files. Eventually, however, I too succumbed to the Imp of the Perverse and wrote some actual Brainfuck programs of my own.
A Brainfuck program has an implicit byte pointer, called "the pointer", which is free to move around within an array of 30000 bytes, initially all set to zero. The pointer itself is initialized to point to the beginning of this array.
The Brainfuck programming language consists of eight commands, each of which is represented as a single character.
> Increment the pointer.
< Decrement the pointer.
+ Increment the byte at the pointer.
- Decrement the byte at the pointer.
. Output the byte at the pointer.
, Input a byte and store it in the byte at the pointer.
[ Jump forward past the matching ] if the byte at the pointer is zero.
] Jump backward to the matching [ unless the byte at the pointer is zero.
The semantics of the Brainfuck commands can also be succinctly expressed in terms of C, as follows (assuming that p has been previously defined as a char*):
> becomes ++p;
< becomes --p;
+ becomes ++*p;
- becomes --*p;
. becomes putchar(*p);
, becomes *p = getchar();
[ becomes while (*p) {
] becomes }