"Visits" is an all-encompassing term for an office visit of any sort: H&Ps, chronic care follow-ups, follow-ups for an acute problem, urgent care, and pap smears.
Diagnostic Tests include any tests performed by a doctor to make a diagnosis, like taking your blood pressure for example.
Procedures are used to actually treat you, and include certain injections or removing minor skin lesions.
There are many important services that may or may not involve an office visit, like getting prescriptions or lab orders.