Tacit knowledge is defined as the kind of knowledge that is difficult to express and thus, transfer. The reason might be because mostly the mind is unconscious (implicit) of the thinking that is done in that domain and also, the reasoning thus implemented. A process (cognition) of the mind can be categorised to be unconscious if the information available to the conscious system is less than that required to make an informed decision. This often results in the inability to justify one's decision especially the "expert" decision.
Expertise refers to the mechanisms underlying the superior achievements of an expert. The superiority might be a result of instruction and practice. The expertise is domain specific and does not transfer to the rational or logical explanations pertaining to a different field. An expert shows a great reduction in the possible solution space of the task. For example, a professional chess player is observed to consider far lesser possibilities (winning) for a move than an amateur. This suggests a refined knowledge representation and decision making. The reason for the rejection of the other moves by a professional cannot be answered by him. This suggests knowledge outside the conscious (explicit) domain of the player. Such tacit knowledge is acquired implicitly through practice and is organised through instruction.