It is in the east side of Shinjuku Station, it's called "Kabukicho" (歌舞伎町) i.e. "Kabuki neighborhood" and is considered the "bad" neighborhood of the metropolis because it features many businesses related to the sex industry; actually it isn't dangerous and it remains one of the most vivid areas of Tokyo. (Incidentally, the name comes from a plan to have a Kabuki theater built there after the war -the theater wasn't built but the name stuck.) One of the most... dynamic branches in Kabukicho is that of the "host/hostess clubs" where female and male customers pay to drink accompanied by pretty boys and girls -how far the "accompaniment" goes is something to be discussed. In the picture, billboards advertising some of these clubs.
(For a bigger version of this picture both in color and black and white, check my "Japan Arekore" set on Flickr)