What is the difference between 3G and 4G ?
3G and 4G refer to different generations (3rd vs. 4th generation) of mobile telecommunications technologies used for cellular phones and other connected mobile devices. Note that the terms refer to sets of standards offering similar data rates. Adding further to the confusion, most of the currently deployed technologies marketed and labeled as "4G" (LTE, WiMax,UMD) are not yet actually compliant to the specifications set forth by the 4G standard.
The main difference between 3G and 4G technologies is the data speed (2-3 Mbps for 3G vs. 5-30 Mbps actual current 4G speeds). Also, 4G LTE uses a completely different radio technology (OFDM vs CDMA) and MIMO (multiple input and output transmitter-receiver antennas).
Note that different telecommunication companies deploy different 4G networks:
AT&T --> 4G LTE and HSPA
T-Mobile --> 4G HSPA+
Sprint, Clearwire --> 4G WiMax (Sprint is moving away from WiMax to LTE)
Verizon, MetroPCS --> 4G LTE
To summarize, 4G simply offers much faster throughput, and uses different modulation than 3G technologies. 4G is currenlty mostly available in metropolitan areas in the US, while 3G networks are more readily available in rural areas throughout the country. You do need a 4G-capable phone/device to take advantage of the service.