NTRIP, CORS and VRS are different things. However, the terms are often used synonymously in our industry.
NTRIP (Network Transport of RTCM via Internet Protocol) and CORS (Continuously Operating Reference Station) are forms of RTK differential correction that are done through the use of a cellular modem and base station network. This means that instead of using the traditional base station and radio to send correction data to a rover, data is sent using the internet to a cellular modem with a data plan. VRS (Virtual Reference Station) is simply a method of RTK correction.