RAM (random access memory) stores the operating system, application programs, and data in current use so that they can be quickly reached by the computer's processor.
RAM is much faster to read from and write to than the other kinds of storage devices like hard disk, floppy disk, and CD-ROM available in a computer. However, the data in RAM stays there only as long as the computer is running. RAM is a volatile memory and when the computer is turned off, RAM loses its data. When the computer is turned on again, operating system and other files are once again loaded into RAM usually from the hard disk. If RAM become filled up, the processor needs to continually go to the hard disk to replace old data in RAM with new data which slows down the computer.
SDRAM, DDR RAM, DDR2 RAM and DDR3 RAM: These are all types of RAMs. SDRAM came first and is the slowest of these RAMs, followed over DDR RAM, then DDR2 RAM and then DDR3 RAM, with each providing greater memory speeds, at least potentially. A computer will use one of these memories according to the memory type supported by the motherboard. Generally, the newer the computer/motherboard, the faster the memory it will support.
The increasing of RAM improves the speed of a computer to an extent. Therefore increasing the RAM is a cheaper and convenient way to increase the speed of your computer to a limited extent.
ROM is an acronym for Read-Only Memory. It refers to computer memory chips containing permanent or semi-permanent data. Unlike RAM, ROM is non-volatile. The contents of ROM remain even after the computer is switched off. A computer comes with a small amount of ROM containing the boot firmware. This consists of a few kilobytes of code that tell the computer what to do when it starts up, e.g., running hardware diagnostics and loading the operating system into RAM. On a PC, the boot firmware is called the BIOS.
Originally, the ROM used to be read-only and physical replacing of ROM chips was required to update programs written on the ROM. The new versions of ROM allow limited rewriting making it possible to upgrade firmware such as the BIOS by using installation software.