This post is excerpted from ITIL http://www.itil.org/custom/glossaren/.
Listed below please find the ITIL definitions for incidents, problems and service requests to describe a ticket type. Properly marked Ticket Type is required for every TWG ticket created.
Incident – An unplanned interruption to an IT service or reduction in the quality of an IT service. Failure of a configuration item that has not yet affected service is also an incident – for example, failure of one disk from a mirror set.
Problem – A cause of one or more incidents. The cause is not usually known at the time a problem record is created, and the problem management process is responsible for further investigation.
Service Request – A formal request from a user for something to be provided – for example, a request for information or advice; to reset a password; or to install a workstation for a new user. Service requests are managed by the request fulfillment process, usually in conjunction with the service desk. Service requests may be linked to a request for change as part of fulfilling the request.