what is the difference between 3g and 4g?

3G is the generic abbreviation for 3rd generation. It is the wireless technology that initially enabled internet browsing over wireless networks.  An example of 3G technology is EV-DO.  4G is the generic  abbreviation for 4^th generation.  It is the wireless technology that was designed to optimize data over wireless networks, improving the speed and efficiency of data delivery.  LTE is one of the technologies that can be used to build a 4G network.