Expansion slots can be used to connect additional peripheral devices such as an external hard drive or monitor. Expansion slots allow additional processors to be connected to the motherboard. Expansion slots can be used to store programs and data.
Memory carries out basic instructions that operate a computer; whereas the hard drive is a long-term storage device. Memory is an electronic component used to store instructions for future use; whereas a hard drive is attached to the motherboard and enables the communication between components.
A computer can only run programs if it has a sufficient amount of memory. (This accurately describes the role of computer memory. How fast a computer can run multiple programs depends on how much memory the computer has.) A PC is the only type of computer that comes with memory modules installed.