Canon 6D VS, T3i

1. What is the difference between a Full Frame Camera and a Crop Sensor?
The difference from full frame and crop sensor is that Full frame gets the whole picture, all that is seen.
Crop Sensor is cropped so you're only getting what is important

2. What are five advantages of the 6D compared to a T3i?
20.2 effective megapixel full frame CMOS sensor 
DIGC 5 + image processor 
11-point AF with one center cross type point 
full hd 1080p video recording
ISO range of 50-102400 (extended)

3. What makes the autofocus of the Canon 5D Mark III better than that of the 6D?

• The EOS 5D Mark III has a 61 point AF array with up to 41 cross-type AF sensors (depending on the maximum aperture of the lens mounted on the camera). Along with the EOS 1D-X, it has Canon’s most advanced autofocus system.
• The EOS 6D has an eleven point autofocus (AF) array with one cross-type AF sensor in the centre.
• The Auto focus on the 5D is similar to the one of the 1D which is really good. The 6D Auto focus is good, just not as good as the 6D.