A Problem is the cause of one or more potential incidents and can generally be identified because one or more incidents have the same symptoms. In GLPI, The Problem allows you to inform, assign and plan, etc., actions or decisions. Problems are accessible in Assistance > Problems.
A problem object can be created :
from the ticket form
in the Problem tab
or directly from the Assistance > Problems menu.
The problem creation form is very similar to the ticket creation form and shares many of the same concepts: Requestor, Observer, Assigned to, Status, Urgency, Impact, Priority, Category.
No, only tickets can be used by the receiver.
Yes, in the same way as tickets, Problems notifications can be customised. Go to Setup > Notifications > Notification templates.