What is the difference between outlook meeting and appointment?

The main difference is that appointments affects only your own calendar and in meetings you can invite others.

In appointments, you don't have the choice to invite others and self is automatically included. Consequently there is no option to Send but you finish it with Save & Close.

For meetings, there is the send option because it is intended to send to participants.

A typical use of appointment is if you want to mark our calendar out of office like here.

Of course if you wanted to, you could arrange a meeting involving only yourself and then it's similar as an appointment.