Battery, AC charger, strap, caps, two cables - one for outputting analog video from the camera, one for USB connection, Several CDs with the manual, EOS software (some useful, some not..:-), body cap (where the lens goes). If you get the 24-105L lens kit you'll also get the caps for both ends of the lens, hood and a cloth pouch for the lens.
H Gardner gave a great answer. I can also say that since I bought this camera I have been blown away. I had a 5d and a 60d and the 6d is amazing. I was the first in my group to have this camera and many that own the 5dmrkII are wanting to sell theirs and get one. The wifi is a huge bonus. Being able to show clients, when not in the studio, the image instantly is priceless. I use PS and not lightroom but have heard that you can configure the wifi to shoot untethered, so if you are a lightroom user this too would be yet another reason.