For those of you living under an anime-culture shunning rock, 'Attack on Titan' is about humainty facing the brink of extinction by the invasion of Titans and a group of militia who fight back against the threat. *cough* Evangelion with giants *cough*
By no means is it a perfect anime, it and the original manga are flawed in some aspects, but Production I.G have made a great adaptation while staying true to the source material. It plays to the sources strengths while patching up the holes, and in fact, I would go as far as to say that it goes surpasses the manga.