The purpose of process is to ensure consistency.

The major difference between program and process is that program is a group of instructions to carry out a specified task whereas the process is a program in execution. While a process is an active entity, a program is considered to be a passive one.

Five states:

new: The process is being created.

ready: The process is waiting to be assigned to a processor.

running: Instructions are being executed.

waiting: The process is waiting for some event to occur.

terminated: The process has finished execution.

Less than five states(like 4 states) can not describe all the situation, while more than five state(like 7 states) is not necessery.