I would say that a 'concept plan" is quite vague whereas a 'master plan' is a plan where various elements of a project are defined but the details are not yet settled. In my opinion, a concept plan of, say, the West Don Lands, would say that the area will be mainly residential and buildings will be less than x floors. A master plan would then be created to define the streets and the blocks and maybe the first few buildings would be outlined or rendered but there would still be places saying' Building 23' or 'possible school'.
A master plan is one which will be executed until the end of a project and a concept plan is a plan which can under go many changes before becoming a master plan.