What is main difference between a RTK GNSS receiver and a traditional GNSS receiver?

The main difference is RTK GNSS takes in the correction stream, but traditional GNSS doesn't.

RTK is short for real time kinematics. A GNSS receiver capable of RTK takes in the normal signals from the Global Navigation Satellite Systems along with a correction stream to achieve 1cm positional accuracy. GNSS includes satellites from GPS (USA), GLONASS (Russia), Beidou (China), and Galileo (Europe). On top of these signals an RTK receiver takes in an RTCM correction stream and then calculates your location with 1cm accuracy in real time.  

RTK is a technique used to improve the accuracy of a GNSS receiver. Traditional GNSS receivers, like the one in a smartphone, could only determine the position with 2-4 meters (7-13 feet) accuracy. RTK can give you centi-meter accuracy.