3G and 4G are classed by speeds. 4G is actually 100mbs, which none of the current technologies attain except LTE-Advanced.
Yet what we call "4G" currently is because they're just faster than 3G (1-2Mbs), and so are really more like 3.75 to 3.9G. HSPA itself can be classed as 3.5G.
If you look deeper however, they can also be classed in terms of technologies. The most important of which is Multiple Access. This is how a station can identify a single mobile from hundreds of others in the same area, read and send to it, while doing the same on hundreds of others.
Classic 2G GSM techs are TDMA or Time Division Multiple Access. This means every terminal is given a tiny sliver of time to send and receive information. In other words, they take turns. This includes the data forms of GSM, which is GPRS and EDGE.
CDMA means Code Division Multiple Access. It means every terminal has a special code and the station recognizes each terminal through this code. The coded transmission is expressed using spread spectrum techniques. Although we associate "CDMA" with CDMA, what we call 3G GSM, aka UMTS, and its related technologies, HSPA, HSDPA and HSPA+, all use W-CDMA or Wide Band CDMA as its air interface While it may use SIMs and GSM base stations, UMTS actually uses a transmission interface method akin to CDMA.
3G "CDMA" as you know it from Verizon, Sprint, etc,. is also known as CDMA-2000. Again, the only thing is shares in name with UMTS is the similarity of the air transmission and multiple access technology, which is code division. This is where you see the 1X, 1XRTT or 1XEV-DO.
LTE and WiMax evolves from CDMA type techs and uses a technique called OFMDA or Orthogonal Frequency Divide Multiple Access. This is a bit difficult to describe, so you can read it here. To make it short, the claimed benefit of OFDMA is higher spectral efficiency, speed and density over CDMA.
Orthogonal frequency-division multiple access - Wikipedia, the free encyclopedia
The second characteristic of 4G is the use of MIMO or Multiple Input Multiple Output antennas. MIMO also increases spectral efficiency and density over traditional antenna design. You would be familiar that some wireless routers, like the ones you see with three antennas sticking out, are using a simple MIMO design.
MIMO - Wikipedia, the free encyclopedia
So basically, both LTE and WiMax uses OFDMA with MIMO techniques.
HSPA+ is basically HSPA added with MIMO.
Some more references here.
WiMAX - Wikipedia, the free encyclopedia