According to Eli (Washington) they live in a world that is a result of some kind of nuclear fallout. It is never fully stated, but bits and pieces are thrown out there for the viewer to fill in the rest. Many people thought this happened because of the bible and religion. So every known copy of the bible was burned. Every known copy except for the one Eli has. So he walks west with it in his bag, for 30 years no less.
The Hughes brothers paint their film in dull burnt grays and browns. It fits the apocalypse feel that the movie if obviously trying to go for. The earth is scorched, the characters search for water and trade boring things as if it were currency. The attention to detail in the set design, costumes and overall atmosphere ire great. They also never let their stars overpower the film. I give them, and the leading actors credit to this.
Washington does excellent in this role. He's getting up their in age and seems to be able to do martial arts with the best of them. In the supporting roles are Gary Oldman, who returns to his true form as the bad guy, and Mila Kunis, who seems to be showing people she actually has acting chops. Oldman doesn't overact in his usual villainous way (The Professional). Here he seems restrained, yet still eerily creepy.
When the credits first started to roll, I was a bit upset with what the Hughes were offering us. But upon further reflection and reading up on what other people's thoughts on the ending were, I've come to appreciate it more. You are set in this world with them and understand it completely. The film doesn't really push many religious overtones on you either.
Check out The Book of Eli, even with it's obvious apocalypse movie clichés, it's a good flick.