Classification of ROMs

Mask-programmable ROM: Look-up array connections are programmed during semiconductor manufacture using a mask. Very fast (>5ns), very dense (~256Mbits), expensive ($20-50K+n*$0.50-20), 2-4week turn-around, low power.
Programmable ROM (PROM): Fuses (sometimes anti-fuses) to program once in the field, using PROM Programmer. Very fast, dense (~ 64Mbits), inexpensive ($2-100), 5min to write, low power.
Erasable Programmable ROM (EPROM): Fuses (sometimes anti-fuses) implemented using floating-gate MOSFETS to program 10K-100K times in the field, using EPROM Programmer. Fast (> 150ns), dense (~ 16Mbits), inexpensive ($2-30), 5min to write, low power, erased by flooding with UV light.
Electrically Erasable Programmable ROM (EEPROM, E2PROM): Same as EPROM, but specific bits may be erased selectively. Fast (> 150ns), dense (~ 16Mbits), inexpensive ($5-50), 5min to write, low power.
Flash Electrically Erasable Programmable ROM (Flash EEPROM, E2PROM): Same as E2PROM, but entire ROM may be erased electrically (no UV). Fast (> 100ns), dense (~ 64Mbits), inexpensive ($5-50), 5min to write, low power. Denser than E2PROM. Very high growth in PCMCIA card/ portable electronics applications.