A work order is the unit everything else rolls up from. A PM schedule generates them automatically on their due date; a fault report from the floor creates one on demand; every labour hour, part used and completion note against an asset's history exists because it was logged against a work order.
The categories that matter most operationally are Preventive (scheduled), Corrective (fixing a known fault) and Safety (statutory or compliance-driven) — separating these makes it possible to see, at a glance, whether a team's week was spent getting ahead of failures or responding to them.