Most ECUs up to 1996 hold their program information and data in small chips called ROMs. These are 28 pin chips 1.5 inches by 0.6 inches. If we want to change either the program information (how the ECU behaves) or the data (usually the fuel and ignition tables), we need to replace the factory ROMs with our own. There are several different types of chip which can be used to replace the factory ROM.

ROMs are read only memory, which cannot be re-used once programmed. e.g. Atmel AT27C256 ROMs are most suitable for the final tune for an engine, which is not going to be altered.

EPROMs are like ROMs except they have a quartz window which allows the EPROM to be erased and reused. Like ROMs, EPROMs are suitable for the final tune.

EEPROMs can be electrically erased and re-programmed, usually much quicker than EPROMs.  e.g. Atmel AT29C256 The advantage of EEPROM is that it is can be programmed in 20 seconds without erasing it first. EEPROMs are most suitable for use when tuning a car, but can be used as the final tune as well.

An EPROM emulator is a device which emulates an EPROM. It is a box of electronics which plugs into the ROM socket on the ECU, and a laptop or PC. The advantage of using an EPROM emulator is that the tuning information in the ECU can be changed quickly without unplugging or swapping ROMs. 

There are two types of EPROM emulators: normal and real-time. Normal EPROM emulators will shut the ECU down while the information in the ROM is updated. Real-time EPROM emulators allow you to change tuning information while the engine is running.