What is GSM, CDMA, TDMA?

Am considering a one year contract with Alltel in NW Florida. I travel
between Panama City and SW Alabama using a state highway that is 20 to
30 miles west of US 231 (where most of the cell towers are located
according to most carrier coverage maps. Alltel has the only full
coverage in the area I travel other than Verizon.

In deciding which phone to purchase (I need only talk capability and
don't need or want pix), I see the terms: GSM, TDMA and CDMA. I see
where some of these are being phased out but do not know what they mean.
Can someone please explain?

Also, is there an external antenna to put on top of a vehicle that will
connect to the stubby antennas on phones?

Thanks in advance.