Since each process is an independent entity with its own program counter and internal state, processes sometime need to interact with other processes.
Sometime, a process may generate some output that is used by some other process as their input.
The diagram given below (state diagram) shows all the three states.
In these three states, a process may be in.
In logical way, the first two states are similar to each other, because in both the cases, the process is willing to run, only in second one, there is temporarily no Central Processing Unit (CPU) available for it. Whereas the third state is different from the first two state, in that, the process can't run even if the Central Processing Unit (CPU) has nothing else to do.