In TimeTap, an appointment is a booking for a service that you offer. As an appointment is made with you, it goes onto your schedule and blocks off your hours of availability to keep anyone else from booking with you at that time. The appointment can be made for any duration of time, but the default length for the appointment will be based on the duration you set for your service.
The service determines the "reason" for the appointment. If you are a Massage Therapist, your service could be called "Hot Stone Massage" and thus when the appointment is made the client sees that he/she is booked for a Hot Stone Massage and will choose between the lengths of massages you offer. If you are a Tutor, your service might be called "One on One Tutoring Session" and, similarly, when you students book an appointment they will choose that from your list of services as the reason for their appointment.
Most of the time, appointments are made with a client who will receive email notifications about the appointment. You can make appointments using the Back Office of TimeTap and not attach a client to it, if you want to.