There are 3 types of diary entries you can create.
The majority of your diary entries will be in the form of a Call - since you will be using the site to schedule and manage planned visits with customers.
The Meeting entry is used to occupy space in the diary to show work related activities not involving any customers. For example, attending the company's annual Sales Conference.
This entry type is to mark the time and dates in the diary when the user is away on leave.
NOTE: This entry type is different from the Leave in a Work Calendar. Work Calendar Leave is company wide, whilst the Leave entry type is user specific.