OnCollisionEnter is not raised when your collider is a trigger as you said. The difference is that objects entering a trigger will not collide with it - quite self-explanatory.
If isTrigger is checked, then OnTriggerEnter is raised and objects don’t collide.
If isTrigger is not checked, then OnCollisionEnter is raised and objects physically collide.
The last part of this page (Collision action matrix) is also very helpful I think, because it shows for different situations if trigger or collision messages are sent