RAM, ROM, Virtual Memory and Caching

All four are types of memory but they differ as to what they can do or store.
RAM(Random Access Memory) is a form of computer memory. You can access any memory cell if you know the row and column that intersect at that cell. RAM causes your computer to feel faster to a point and it eliminates the need to "Swap" out programs. RAM will be lost every time the computer shuts down.
http://computer.howstuffworks.com/ram.htm
ROM(Read-Only Memory) is a circuit with preprogrammed data on it. They are used in most electronics. ROM can not be edited, if a mistake is made you must start all over with ROM.
http://computer.howstuffworks.com/rom.htm
Caching is a memory system. It is used to access the memory of the main memory faster that normal. An L2 cache access' the main memory 2x faster than normal and an L1 cache access' the main memory 3.5x faster than the L2 cache. Caches are used between subsystems of the computer to improve performance. This is also used to cache a slow medium like the internet.
http://computer.howstuffworks.com/cache3.htm
Virtual Memory is part of tha RAM which looks for unused memory and copies applications onto the hard disk which frees up space on the RAM to load a new application. The copying of the applications happens automatically and makes the computer feel like it has more memory than it really does. This is also an economic benefit because hard disk space is cheaper than RAM chips.
http://computer.howstuffworks.com/virtual-memory.htm