4G-LTE is a wireless technology capable of download speeds up to 10 times faster than 3G. That's roughly the speed of your home broadband network.
Carriers are rolling out 4G over the next few years, and they're in different stages of their expansion. Verizon currently has 371 markets covered nationwide, covering 75% of the U.S population. AT&T has 65 markets, covering 24% of the population. Sprint covers just 19 markets. Those numbers are in constant flux: All of the carriers plan to significantly increase their 4G coverage fairly rapidly.
AT&T says it will have 4G LTE in more than 100 cities by the end of 2012. Sprint says it will have its network available in 100 additional cities "in the coming months." Verizon plans to have 400 markets covered by the end of the year. Some metro areas where LTE is partially deployed do not yet appear on carriers' official coverage maps.
Eventually, all three nationwide iPhone carriers say their 4G coverage will be as pervasive as 3G. Until then, users will find their iPhones shifting in and out of 4G. When your phone leaves a 4G zone, it will seamlessly move to 3G as a backup.