Multiprogramming - A computer running more than one program at a time (like running Excel and Firefox simultaneously)
Multiprocessing - A computer using more than one CPU at a time
Multiprogramming - More than one task/program/job/process can reside into the main memory at one point of time. This ability of the OS is called multiprogramming.
Multitasking: More than one task/program/job/process can reside into the same CPU at one point of time. This ability of the OS is called multitasking.
Multiusers System - a computer system in which multiple terminals connect to a host computer that handles processing tasks.