One of the basic things that differentiates GSM (based on TDMA) and CDMA is the way the carve up bandwidth.
Each bit of radio spectrum used by a handset has to be shared with
other people in the area. It is more or less the same as multiplexing
for normal data land lines.
The major difference between GSM/TDMA and CDMA is in the way they divide up those signals between multiple users.
GSM/TDMA uses a Time Division method. TDMA, in fact, stands for Time
Division Multiple Access. Simply put, this means that each device on
the local network is allocated a time slice where it "owns" the
bandwidth, and it can send/receive its data.
So lets just pick a number and say there are 30 available time slices
in a given cycle. Each phone would then get 1/30th of every cycle that
it could send and receive data (aka, voice).
CDMA uses a different method, called Code Division Multiple Access. The
specifics of how it breaks the cycle up are beyond me, but how it works
out is that the phones only get a slice of the bandwidth cycle when
they actually need one. So if you are not talking, and the other person
is not talking, nothing is transmitted.
With GSM/TDMA, each phone is transmitting and receiving during its slices of the bandwidth cycle, whether it needs it or not.
Since most coversations are comprised largely of silence, the end
result is that CDMA phones have to transmit less data. They don't have
to send silence, like GSM/TDMA phones do.
This means a few things. More CDMA calls can be fit into a given amount
of frequency spectrum (ie. it is more efficient for the network), less
radiation is being created from the phone towards the user (you only
get radiation when you are talking, basically), and battery power is
conserved since the handset only transmits when it actually has
something to send.
There are other differences, too, that I can't get into. For one, it is
harder to implement a CDMA network. The tower placement is more
difficult. Dealing with hills is more difficult than with GSM/TDMA.
Things like that.
But in general, CDMA is vastly superior technology. Not surprising
since Qualcomm's version of it (that which is used in CDMA and WCDMA
phones) is newer technology, even if the basics were in use by the US
military as far back as the 40s.
The important thing for me, though, is the SIM card. Had CDMA
implemented the use of a SIM card (something it very easily could have
done), then the North American market would be very different today,
and there would be more CDMA networks in other countries. The SIM card
allows people to easily switch phones, and that helps the market.
Anyway, I hope that gives you a basic overview.