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ROM means Read-Only-Memory. ROM is a form of permanent storage. This is information that is pre-stored in your computer so that when your computer is first turned on, it doesn't have to load this information from the hard drive.
ROM is pre-programmed with its information in the manufacturing process - once made it can never be changed. Because you can not change any of the information on a ROM, it is mostly useful to designers for storing parts of the Operating System that have no need to change. This code or data can be used from ROM, which saves the computer from having to use RAM to achieve the same function (and ROM is cheaper). Macs have about 4 Megabytes (4 million characters) of ROM, PC's have 16 Kilobytes (16 thousands characters).
RAM is often faster than ROM, so sometimes people "shadow" or "mirror" the ROM -- they copy it into RAM and then always access the RAM version. This is wasteful of RAM, but since RAM is faster, some people are willing to trade performance for space. This may sound funny, because why wouldn't you just store what was on ROM on the hard-drive? Some people do -- but then it has to load all that into Memory first -- where copying from ROM to RAM is much faster.
TYPES OF ROMS
There are a few different types of ROM. Primarily users care about just plain old ROM, but engineers are very specific about the type of ROM used. Most computers use plain old ROM, but there are small amounts of the other types of ROM's and so explaining the differences may help users.
PROM means Programmable-Read-Only-Memory. PROM is a form of permanent storage that you can write to, but just once. The process of making ROMs has to be done at a chip-manufacturing plant, but PROM's let many companies make their own ROM's without a long manufacturing process. PROM's work by putting the chips in a special "burner", and sending the information to the chip. The chip starts out as all 1's, and the burner can pop specific 1's into a zero state -- the results are that any value can be stored permanently on a PROM, and PROM's can be used in place of ROMs. PROM's cost more, so are used in small volume manufacturing or in testing stages, on data that will rarely change (every week, month or year).EPROM means Erasable-Programmable-Read-Only-Memory. EPROM is a form of PROM that you can not only write to, but erase. Erasing is achieved with a little window on top of the chip, that allows programmers to put it under an intense UV (Ultraviolet) light, that will reset all the burned values back to their initial state. Other than that, they work just like any other PROM. EPROM's cost more than PROM's but you can reuse an EPROM many times - so they are used in testing or engineering labs where the data is likely to change a fair amount (daily or weekly).
EEPROM means Electrically-Erasable-Programmable-Read-Only-Memory. EEPROM is a form of EPROM that you can erase electrically (no UV lamps). Erasing is achieved by sending special electrical signals to the chip (usually higher voltage). Other than that, they work just like any other EPROM. With special circuits added to your design, you can have a ROM that can change whenever you want it to. EEPROM's have limited life-spans -- you can write to them a few thousand (or million) times - but if you were using them as main memory (like RAM), a few million writes could mean they would last for only a few seconds. EEPROM's also erase slower than RAM, and have to have special logic to support them. EEPROM's are good for saving settings and preferences or code that will be changes maybe once or twice a day (or week), but not for data much more frequent than that. Modems often have EEPROM in them (or similar device) to store your settings in a permanent way -- but now days something called NV-RAM does a similar function and costs less.
FLASHROM means that you can erase the ROM in a "Flash". It is a form of EEPROM but may be manufactured a little differently. Erasing is achieved usually in banks of memory (chunks), instead of character my character like in EEPROM, this helps make it cheaper to manufacture.
Conclusion
I hope this gives you an understanding of what ROM is, and the diferent types of ROM.
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