It was the first-generation technology, using FDMA, which has inherent problems with capacity and security. Each cell has about 1000 channels or frequencies in the 824~849MHz range, and must use a different set than neighboring cells to avoid interference. The AMPS band was taken from the same 806~890MHz frequency band which was originally UHF TV channels 70~83. (The remainder went mostly to public safety use, after the few TV stations using those channels were required to move.)
Later, AMPS was upgraded to D-AMPS using digital TDMA, called second-generation or 2G. This increased capacity because voice can be digitally compressed into a much smaller bandwidth. It also made calls more secure because analog scanners could not access digital signals, and new scanners in the U.S were prohibited by the FCC from accessing cellphone frequencies.
The latest third-generation technology or 3G is PCS, which uses the 1900MHz band in the Americas.
Total Access Communication System or TACS is the European version of AMPS. ETACS was an extended version of TACS with more channels. TACS and ETACS are now obsolete in Europe, having been replaced by the more scalable and all-digital GSM system.