What's a Barker?

So I’ve searched high and low, looking for the answer. What is a barker? There was obviously a guild devoted to it seeing as they produced a play in the York cycle! The best I’ve come up with is this. “Someone who stands in front of a show (as at a carnival) and gives a loud colorful sales talk to potential customers” This definition sounds close to what I’d image a barker to be. Something akin to this…

Pirelli's Miracle Elixer

[Is this a Baker?]

"Barker." WordNet. Princeton University. Web. <http://wordnetweb.princeton.edu/perl/webwn?s=barker>.