Brainfuck is sort of the poster child for esolangs.
Brainfuck operates on an array of memory cells, each initially set to zero. There is a pointer, initially pointing to the first memory cell. You then increment and decrement along the memory cells, and are able to preform limited actions on cells.
- move the pointer right, >
- move the pointer left, <
- increment the memory cell at pointer, +
- decrement the memory cell at pointer, -
- output character signified by the memory cell at pointer, .
- input character and store it in the memory cell at pointer, ,
- jump past the matching ] if the cell at the pointer is 0, [
- jump back to the matching [ if the cell at the pointer is nonzero, ]
All characters other than ><+-., are typically considered comments and ignored.