2channel (2ちゃんねる, ni channeru; 2ch for short) - known as 5channel (5ちゃんねる, go channeru; 5ch for short) since October of 2012.
A Japanese internet forum, often thought to be the largest in the world. It has over 600 boards covering a wide range of topics, as their slogan ('From "hacking" to "side dishes for tonight's dinner"') would suggest.
2ch/5ch plays a large part in Japanese internet culture and has created a culture and language in and of itself. ASCII and Shift-JIS art is popular there and many meme/neta and characters have been created based on it. In addition, several internet celebrities have been born there, as have many collaborative projects and pranks (ie: Densha Otoko, Buront, Kasane Teto and the Vippaloids.)
It is important to note that 2ch/5ch is not an image board - it is a text board. Do not confuse with 2chan, also known as the Futaba channel.