Akihabara. Otaku heaven full of figures, gundams, electronics, maid cafes, candy girls and of course - ero stuff.
If you dont know by now, "ero" = short form of "erotic" = adult. Eroge is basically adult games - you may know of them as "hentai games".
Doujinshi are self published manga or novels - many of which are of the ero variety.
This was taken in Akiba last week. Otaku generally want to get hold of their eroge on the day of release and what they will do is come to a shop like this to reserve their games. On the day of release, you will see lines of otaku waiting outside the shop early in the morning.
Some of the following photos are NSFW.