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.

All characters other than ><+-.,[] are typically considered comments and ignored.
irc bot written in brainfuck

backlinks: esolangs.html languages.html