From the Ask Tom mailbag:

Question:
What’s the difference between a process and a system?

Response:
A process is the way we get something done. A process is a shift from the haphazard, often the backbone of a discipline or a set of instructions. A process can take the form of a checklist, often has an order or sequence to its steps, like do this first, then do that.

A system is more robust than a process. A system has a defined start and a finish. A system also has a series of steps, but there are different relationships between the steps. In a system, some steps can be done at the same time (concurrent steps). Some steps depend on other steps to be completed first, before the current step can begin (dependent steps). Some steps run at different speeds and can bottleneck other steps (constraints). Some steps may only be required under specific conditions (conditional steps). Some steps may loop back to previous steps and repeat (iterative steps). A system looks at specific conditions, finds the similarities and, by treating them the same, produces a consistent outcome. A system looks at specific conditions, finds the differences and, by treating them differently accommodates a range of variables.

Looking at levels of work, a process may more accurately be used at S-II, while a system is more likely required at S-III.