Service Request Tickets 
Service request tickets cover requests for general maintenance and common IT services. Some examples include password resets and requests for new hardware or software. When a user files a service request ticket, the service desk software might also point them to a knowledge base that offers help on common issues.

Incident Tickets
Incident tickets are filed in response to unplanned interruptions to IT services. The end goal of incident tickets is to resolve the issue quickly and restore normal operations. An internet outage, an enterprise app crashing, or any other disruptions to routine IT services are typical cases for incident tickets.

Problem Tickets
Problem tickets are a response to incident tickets. They're used to track the progress of finding the root cause of an incident. For instance, a problem ticket is an appropriate place to document the root cause analysis if an internet outage keeps recurring. Problem tickets are more often filed by IT staff rather than from direct requests by their co-workers.

Change Request Tickets
Change request tickets are for requests to modify or replace an IT service or product. If, for example, a WiFi router is suffering from poor reception in a given area, a user might file a change request to ask for a replacement. Or if an enterprise app is producing an unsatisfactory result, a change request is the appropriate way to ask for a resolution. A separate approval process may be required for requests requiring significant expense.