"Visits" is an all-encompassing term for an office visit of any sort: H&Ps, follow-ups, urgent care, and pap smears.
Diagnostic Procedures are relatively self-explanatory. They include any procedures performed by a doctor to make a diagnosis, like taking your blood pressure for example.
Therapeutic procedures are procedures done to actually treat you, such as certain injections or removing sutures.
There are many important services that may or may not involve an office visit, like getting prescriptions or lab orders.