Nijiura (二次元裏 or 虹裏 Nijigen ura) is a set of image boards on Futaba Channel (2chan), roughly equivalent to 4chan's /b/, except in Japanese and without all the fucking idiots. Oh, and there's more than one board, and they have a lot more anime and a lot less camwhoring. Okay, it's really not like 4chan at all, but it used to be, back when 4chan was new. Kind of.
There are actually six /b/ boards on 2chan, although the only two listed in the board index are may and jun. Comparing the various boards is not unlike comparing the various English /b/ boards (e.g. 4chan, 7chan, ko-chan, iichan, etc.): although they exhibit a large number of similarities in overall content and memes tend to spread from one board to another, the differences are still quite noticeable. Looking solely at may and jun, for example, jun tends to have quite a bit more hentai and loli content, whereas may is somewhat more focused on real-world stuff and has quite a bit less porn.
Most Japanese boards (on Futaba Channel, Ni-channel, and elsewhere) use "nameless" (名無し Nanashi) as their equivalent for Anonymous. However, in the early days of 2chan, a number of posters copied a troll posting with the name Toshiaki (としあき), and this ended up as the default name on many boards, and has come to be a generic name for Nijiura regulars. The -aki suffix has been adapted to other words, such as "Joshiaki" (女子あき) for female posters (note that Japan does not adhere to the "no women on the internets" meme), or "Yuriaki" (ゆりあき) as the default name on the yuri board.
Additionally, some of the boards have names disabled altogether, so there is no Toshiaki in sight. Posters on these boards are often referred to as 「」 ("kakkou", or "brackets"). This is somehow also used as a suffix.