What is 3G?
The G of course means generation. Cell phones and the systems are classified by the generation they belong to. Third generation (3G) phones were developed in the late 1990s and 2000s. The goal was to improved the data capability and speed. 3G phones were being defined by the Third Generation Partnership Project (3GPP) and later standardized by the ITU-T. Generally known as Universal Mobile Telecomunications System (UMTS), this 3G system is based on wideband CDMA that operates in 5 MHz of bandwidth and can produce download data rates of typically 384 kb/s under normal conditions and up to 2 Mb/s in some instances. Another 3G standard, (cdma2000), was developed by Qualcomm. It used 1.25 MHz band to produce data rate to 2 Mb/s. Another version of cdma2000 is a improved IS-95 version. It is a 3GPP2 Standard. It can transmits the data at a rate to 153 kb/s and up to 2 Mb/s in some cases.3G phone standards have expanded and enhanced to further expand data speed and capacity. The WCDMA phones have been added high speed packet access (HSPA) that use higher level QAM modulation to get speeds up to 21 or 42 Mb/s downlink (cell site to phone) and up to 7 and/or 14 Mb/s uplink (phone to cell site). AT&T and T-Mobile uses HSPA technology. The cdma2000 phones added the 1xRTT as well as Rev. A and Rev B modifications that boosts up speed as well. Verizon and Sprint uses cdma2000 3G standard technology. Virtually all standards and smartphones models and most tablets still use some form of 3G.
What is 4G?
The fourth generation has been defined but we are not in it yet. Yes, many if not most of mobile carriers and the various phone and equipment manufacturers actually advertise 4G now. The formal definition of the 4G as declared by the 3GPP and the ITU-T is something called Long Term Evolution-Advanced (LTE-A). The standard has not been fully completed but basically it is an improved and enhanced version of LTE that uses wider bandwidth channels and a greater number of MIMO antennas. The theoretical upper (datarate) is 1 Gb/s. That remained to be seen in practice.
As for what the various companies are calling 4G, Verizon says that LTE network is 4G. AT&T promote their LTE and HSPA networks as 4G. T-Mobile indicates that their HSPA+ networks are 4G. Furthermore Sprint and Clearwire says that their WiMAX network is 4G. As mentioned, WiMAX is actually being defined as a 3G technology by ITU-T like LTE.
Since both LTE and WiMAX are completely different technologically from previously defined WCDMA/cdma2000 3G, it seemed appropriate to say LTE and WiMAX are 4G. Both are step up in data speed. It all depend on your interpretation and acceptance of what 4G is. If carriers are calling LTE 4G now, will they called LTE-Advanced 5G. If it is good marketing, they may.