An appointment is an activity that involves only you. Use an appointment when you want to show that you're busy but no one else in your organization is involved. You might enter an appointment to block out time in your calendar when you need to meet with a client, visit the dentist, or research a project you're working on.
A meeting is similar to an appointment. The difference is that you invite other people to it.
An all day event is an activity that lasts all day long but doesn't block out time in your Calendar. Use an all day event when you want to show an activity that occurs on a specific day without having it clutter up all of the time slots in your calendar. Here are some types of entry for which you might use an all day event:
A holiday
A recurring annual event such as a birthday or anniversary
Other items that last all day, such as your vacation schedule