ROM Emulators "look" like a ROM to whatever you connect them to. In most cases here, that would normally be an ECU. The most important feature of a Rom Emulator compared to an EPROM or even FLASH-ROM is that you can change the data that a ROM Emulator contains on the fly, while it is running. Most often, this is accomplished using SRAM/NVSRAM coupled with a bunch of logic gates or a FPGA to avoid crunching data.

the Xtronics Romulator is the most commonly heard of Rom Emulator as it is endorsed by Hon Data for use with their products.