PROM stands for
'PROGRAMMABLE READ ONLY MEMORY'
or ONE TIME PROGRAMMABLE (OTP)

EPROM stands for
ERASABLE PROM
or MULTI TIME PROGRAMMABLE (MTP)
most often are they erased by using UV-light.

EEPROM stands for
ELECTRICAL ERASABLE PROM
sometimes you need a higher voltage to erase the ROM.

FLASH stands for
..... don't know actually ????
Well, it's just an other technology (not with "hot electrons")
They are VERY flexible and reprogrammable IN-SYSTEM ...
They have an internal state machine that allows any user to read / write or erase blocks or cells.

PSD has some nice FPGA / FLASH / RAM devices combined in one device. Take a look at this page at your left side (bottom)

bert Regards