Memory

RAM

It is common for the modern PC to have 1GB of memory or even more. A lot of people know that more memory will mean a faster computer. However not many people know why. Primary memory , or Random Access Memory (RAM), stores what the user is currently working on. It is much faster than the hard drive, which is known as secondary storage. The CPU must load instructions and data into its registers in order to work on them. As such what it normally does is load programs and data into primary memory. It then loads it from main memory into the registers. So at any given time your data and programs currently running will be stored in RAM.
When the PC is switched off any information stored in main memory is lost. This is why it is important to save work to secondary storage.
Primary memory is much faster than secondary memory but is still much slower than the CPU. As such the CPU needs to use caching techniques to ensure the system runs as fast as it possibly can. Also virtual memory is used to expand the amount of primary memory available

ROM

It is easy to confuse RAM and ROM together. Read only memory or ROM stores essential system software needed for the running of the PC. It will store software which will help start the PC up and also ensure its smooth running. It does not store the operating system. The OS tends to be stored on the hard drive. It also can not save settings (as it is read only!)