It is a rather long suspense/action/thriller/sci-fi manga by the quite superior mangaka Naoki Urasawa who also wrote Monster and is currently writing Pluto.
It is about a group of friends and their battle with a ghost from their childhood, a masked and mysterious cult leader named "Friend" who has grandiose designs.
It is a very good manga. A serious page turner that I read over the course of two days. It has a complex and well written narrative, not to mention a great cast of villains. "Friend" is also quite creepy.
However, two major faults I see it as having are:
It WILL leave you hanging on its secrets. It usually does this by skipping to another narrative as a character realizes or sees something. It didn't frustrate me, and I wouldn't want anything revealed earlier than it was but it might frustrate you.
Another major complaint is that it drags out a bit too long in the end or that the end is somewhat extraneous (and that the manga could have ended earlier). Both of these are true, but the final arc is still enjoyable.