Zero-rating (also called toll-free data or sponsored data) is the practice of mobile network operators (MNO) and mobile virtual network operators (MVNO) to not charge end customers for a well defined volume of data by specific applications or internet services via the MNO's mobile network in limited or metered data plans and tariffs.[1] it allows customers to use data services like video streaming, which otherwise might result in significant charges depending on their data plans and volume caps. This has especially become an option to market 4G networks, but has also been used in the past for SMS or other content services.
Source: http://en.wikipedia.org/wiki/Zero-rating