Just thought I'd make it clear to the OP:
PROM = programmable ROM, otherwise known as OTP (one-time-programmable). You can write to it once and no more.
EPROM = erasable PROM. Has limited write cycles (think: limited number of times you can write to it). Erasing them requires use of a UV light and 1-2 hours of time. More expensive than PROM.
EEPROM = electronically-erasable PROM. Has limited write cycles like an EPROM. Erasing them can be done purely through software (usually the same software which does the writing). More expensive than EPROM.
FlashROM = flash-based EEPROM. Functionally the same as an EEPROM, with greater life expectancy (more write cycles). AFAIK is faster during writes/erasing. More expensive than non-flash EEPROM.
Hope this helps. To others more familiar with hardware stuffs -- if I got any of this wrong, please do correct me!