Pretty much what Scott said. The main guy who appeals to your target demography/market. So with WWF Attitude Era, Stone Cold was the face of the company for most of the time. With the transition to Attitude Era and RA era, The Rock became the face (01-03)
Face and heel meant nothing because The Rock was the face of the company until he retired in 03 despite being heel for his 02-03 run. He was the guy everyone came to see even as Hollywood Rock.
Same thing with the RA era. Brock and Angle were the faces of the company. They TRIED to get HHH to be the "heel face" but Smackdown was shitting on Raw's soul at the time so they had to accept it.
With the Cena thing, even if other people are more over than Cena he fits the TARGET DEMOGRAPH of WWE's product. So he's been the face of he company since 2005, and theyve even adjusted his character to decline with it.
More often than not you can choose who you have as the face of the company. But usually it works hand and glove and doesn't have to be forced. A prerequisite, of course, would have to be that everyone knows him even if they dont watch wrestling. Daniel Bryan could get to that as soon as this year. I had friends who I didn't know gave a shit about wrestling anymore mention him the last year and 1/2. If he gets to beat Lesnar and is promoted as THE guy, he'd be THE guy.
by 
Subject: The Face Of.. 
