They are all basically generations of technology for how your phone speaks to a cell phone tower.

The original 1G phones used a system called FDMA, Frequency Division Multiple Access. In this system each user who makes a call connects to the cellphone tower using a specific frequency. So, 1 user, 1 frequency. This was used very early on, and as cellphone use began to expand it was necessary to find a way to expand the radio spectrum since there is a limited number of frequencies available.

That introduced 2G phones which used TDMA, or Time division multiple access. This scheme divided the frequency into smaller time slots. The idea is that multiple users can take a "slice" of the frequency time slot and then the tower will move them around the time slots to transfer data. Once again, they needed to make better use of the frequencies as more users got cell phones.

Meet 3G, which uses CDMA or code division multiple access. This further split the frequencies into small blocks. The key to this concept is that when two users connect to the same frequency (the person you are talking to on the other side) they have a specific Code that corresponds to their segment of frequency. Think of it like this. You are in a room with 200 people, but everyone is speaking jibberish besides the person you are directly speaking to. They understand the "code" so you can understand them and block everyone else out.

I don't know as much about 4G, but it stands for Long-term evolution, and is only available in certain parts of the world. It is essentially a much more efficient way of communicating to the tower using a similar architecture, which provides the ability to get such fast speeds.