I have become very familiar with these Skype interviews having given three of them already. The first thing to understand is that if the Professor wants to meet you, then your application must have impressed him and the grad school as a whole. The basic questions they asked me were about my internship (at ETH Zurich), my career goals, which other Universities I have applied and what preference do I give to that grad school. If you have already got some offers, they may ask you how you rate that