I had a skype interview last year for the graduate program I am in now. It was not at Columbia, but it is my understanding that skype interviews are pretty much the same. It is definitely good news, means that they are interested in you, and you should treat it as a regular interview. Make sure you are prepared for their questions, such as what you want to do, why Columbia specifically, why engineering, etc. Also make sure you do the Skype interview in a quiet room and let others know not to disturb you, I had my roommate walk into the background of mine (despite all the signs I posted around our dorm!) and it was a bit embarrassing. Nonetheless, just be prepared to answer some questions, have some questions to ask them, and relax! Good luck!