Hi.

A simplistic way to look at it is:

ROM is a measure, usually of how far a joint moves

Mobilisation means simply to "move". It can apply to a joint (usually in musculoskeletal physio) or it can apply to a person walking etc.

ROM exercises are simply exercises that move a joint through its ROM with a view to maintain the ROM or to increase it with a stretch component at the end.

A lot of physios tend to use mobilisation to mean an accessory movement applied to a joint.

If you want more info, just ask!