The difference is that ROMs store data -- zeros and ones. Gate Arrays store logic connections. Gate Arrays are capable of performing logical functions very quickly compared to software. Thus FPGAs are potentially a tool for programming very fast data conversion algorithms on the fly. FPGAs are often used where field programmability or speed are more important than cost. There are small inexpensive FPGAs that are used in some consumer devices. Often FPGAs are used in development of a technology that is later transferred to cheaper conventional ICs for production.
The actual techniques used to implement logic in FPGAs rather complex and specialized. They may use fast programmable lookup tables, programmable gates with multiple inputs, or both, to implement logic.
FPGA is a Xilinx trademark. Other manufacturers use other names for similar devices -- PLD, PLA, etc. However the acronym FPGA is often used generically to describe the technology.
While (F)PGAs have been available for many years, they have been primarily used as custom "glue" logic to tack together large scale integrated circuits. However FPGAs have recently become available in very large configurations -- up to 4,000,000,000 gates. This is large enough, for example, to conceive of using programs for two different CPUs and simply reprogramming the FPGA on the fly to emulate one CPU or the other as needed. It is also conceivable that FPGAs could be used to program one set of hardware to connect to telephones in a variety of countries by reprogramming the on chip logic. FPGAs could also be useful in a variety of security and data locking schemes.
It is not clear that large FPGAs will actually be used in exotic ways. They could turn out to be a technology forever in search of applications.
Note: I found this article while cleaning up comments on the Glossary. It's possible that there is a second copy of the article in the Glossary with a different title as I could have changed the title after it was pointed out to me that FPGA is actually a trademarked name.
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