Workflow Rule VS Approval Process

Workflow Rule	Approval Process
Automatic (triggered when record matches criteria)	Manual. Can only be invoked if record matches criteria.
No interaction required (time-based workflow rules are transparent as well).	Each step of the approval process requires interaction from a user (approve/reject).
Uses workflow actions	Uses workflow actions
Does not lock the record under any situation	Locks the record during the approval process.
Changes to record tracked normally (as if user initiating the workflow rule made the change)	Approval process audit history listed in separate related list on object