Paraphrasing wikipedia:
Multiprogramming – A computer running more than one program at a time (like running Excel and Firefox simultaneously)
http://en.wikipedia.org/wiki/Multiprogramming
Multiprocessing – A computer using more than one CPU at a time
http://en.wikipedia.org/wiki/Multiprocessing
Multitasking – Tasks sharing a common resource (like 1 CPU)
http://en.wikipedia.org/wiki/Computer_multitasking#Multithreading
- Thus, something like multithreading is an extension of multitasking.