The memory chips store information, temporarily, for short term use. A PC's memory is an entirely different thing from the hard disk "memory". The hard disk stores information "permanently" for long term use.
A PC's memory only contains information when the PC is on. When the PC is turned off, the information in the memory chips disappears. The information in memory is similar to a thought, it gets replaced when you start thinking about something else. Hard disk memory is like writing down the information and storing it in a filing cabinet.