When you run a program, the CPU looks for it on the hard disk and transfers a copy into the RAM. RAM (Random Access Memory) is temporary or volatile, it holds data while your PC is working on it, but loses this data when the power is switched off. 

However, ROM (Read Only Memory) is permanent and contains instruction needed by the CPU; the BIOS (Basic Input Output System) uses ROM to control communication with peripherals, e.g. disk drives.

The amount of RAM determines the number of programs you can run simultaneously and how fast they operate. It can be expanded by adding extra RAM modules.