Popular cellular phone operators in India use GSM or the CDMA technology to provide voice and data services. Here I would like to explain these terms, what they mean, their implications to lay-persons who do not understand technical jargon.
GSM : GSM stands for Global System for Mobile Communications. Just like computers, mobile phones have evolved over time. There were first generation mobile phones in the 70's, there are 2nd generation mobile phones in the 80's and 90's, and now there are 3rd gen phones which are about to enter the Indian market. GSM is called a 2nd generation, or 2G communications technology.
Now, GSM makes use of two principles. The first called Time division Multiplexing is very simple. Let's say that one of your friends possesses the new and sleek Apple's IPhone. Everyone wants to try a hand at your new accessory. What do you'll do? You lend it to your friend Tim for sometime, then you let John listen to music on it, then you let Linda check her e-mails on it, and finally you let Nancy search on google. So what did you do ? You allowed each of your friends to share you IPhone for sometime. That is, you allowed you IPhone to be shared in time.
On the same lines, in GSM, the radio frequency say 890 Mhz is shared by different users in time. This means if user A, B, C and D all talk at the same time. You assign the 890 Mhz frequency to A for some time and allow him to talk, then you assign 890 band to B for sometime to speak, then to C , and finally to D, before coming back to A. So the process continues in a round robin fashion, as long as A, B, C, and D want to talk. This way many users talk at same time on the same frequency. This has to be done, because as we now frequency or Bandwidth is a scarce resource and is not available in plentiful, so it must be shared.
Now the second principle that GSM uses is Frequency Division Multiplex. In Frequency Division Multiplex, users A, B, C and D, all use different frequency say 890, 900, 910 , 920 for their respective communications. A very good example of this is Radio broadcasting. Because all the radio operators like Rad FM, Go FM, Radio Mirchi want to operate in the same area, they use different frequencies for communication 91.0FM, 93.5FM, 94.6 FM, 108FM. So to listen to different communications, you have to tune in the receiver set to different frequencies.
Now, GSM uses a combination of TDMA and FDMA. This means that users A and B are not only sharing the channel in time but also frequency. This means that user A is on the channel 890Mhz for 2 seconds, then jumps to 900Mhz channel for the next to seconds, then jumps to 910Mhz for the next 2 seconds and so on... Thus, each user is uses a different frequency at different time slots. This is called Frequency Hopping.