A timer job runs in a specific Windows service for SharePoint Server to automate common SharePoint tasks.
"A timer job runs in a specific Windows service for SharePoint Server. Timer jobs also perform infrastructure tasks for the Timer service, such as clearing the timer job history and recycling the Timer service; and tasks for Web applications, such as sending e-mail alerts. A timer job contains a definition of the service to run and specifies how frequently the service is started. The SharePoint 2010 Timer service (SPTimerv4) runs timer jobs. Many features in SharePoint Server rely on timer jobs to run services according to a schedule." **From technet.microsoft.com