Describe the life circle of process

When a process is created, it is in a New state.

Then if it is admitted to be assigned to CPU, it enters the Ready state.

Once CPU is available, the scheduler will dispatch the process to CPU. This is the Running state.

A this moment, a interrupt could make the process return to Ready state.

Or the process itself invoking I/O or other events, it's the Waiting state. When I/O or event required is complete, the process returns to Ready state.

If nothing else while Running, the process finished execution, changing to Terminated state.

Why each process must have one PCB

PCB contains all the infomation that a process will need for Process Scheduling. Without a PCB, process management will be difficult and inefficient.