A Booru is an image server concept first developed off of something called the Danbooru engine. These small PHP-MySQL servers have been used to host gigantic repositories of pictures, usually Japanese hentai and manga. Support for the Danbooru engine has been discontinued within the past few years, as have most of its clones and substitutes, making the base code quite difficult to find and install on one's own system.
However, one was found called Shimmie2, which was uploaded to this site. Its creators designed a base look for this server, but also included the functionality and appearance of the old Danbooru style.
NOTE: Shimmie2 is currently unavailable due to ongoing upgrades to the system.