Cake -- it looks like coke in a coal furnace: a charcoal colored, charcoal looking "coating" on the inside walls of the pipe bowl.
This "cake" is a good thing, IMO, because it does help insulate the wood itself from the considerable heat of the burning tobacco; cake can and will mitigate spider webbing, wood cracking and burnouts.
As noted by others in this thread, the thickness of the cake is subjective, having 'way too thin or worse, no cake prevents the insulation benefits I've talked about. Having the cake too thick is just as bad -- the cake can actually cause the wood to crack, not to mention the shirinking of the tobacco chamber by the thick cake leaving less and less room for tobacco, neither of which is a good thing.
I like a dime thickness. I use a dull pipe knife to keep it that way; I haven't touched a reamer in years. Some like a nickel thickness. You decide what you like, but I do advise you to have at least some cake inside the bowls of your wood pipes.
As for Meerschaums, no cake. None. Zip. Clean the inside of the meerschaum bowl thoroughly after each smoke.