Computer memory is a term used to describe the physical storage devices used to store data on a computer. The most common type of computer memory is RAM, which stands for Random Access Memory.
RAM is a volatile memory, which means it is temporary and loses its information when you turn off your computer. It is used to store programs that are being executed.
ROM is another form of computer memory that is used in computers. The first personal computers did not have RAM, but they had a permanent memory device called a PROM. A PROM was used to store instructions for the computer. The computer would run the program when it turned on.
A harddisk and SSD is a non volatile memory. It will keep the data even when your computer is turned off. Non-volatile memory is used to store the information about your files that are already saved on your computer.