Timer Job in MOSS

Many different types of applications require some variation of a scheduled process to run. These processes are used for complex calculations, notifications, and data validation checks, among many other tasks. Windows SharePoint Services is no exception. To return relevant and timely results to users' search queries, the content within a server farm must be indexed ahead of time. This indexing is performed at scheduled intervals. Another example might be sending nightly or weekly e-mail messages to users who want to be notified when changes occur in a SharePoint list. These scheduled tasks are handled by the SharePoint Timer service, a Windows service that is set up as part of the installation process.
The SharePoint Timer service is similar to tasks that we can create in any version of Windows by using the Task Scheduler application. The major benefits of using the SharePoint Timer service compared with Windows Task Scheduler jobs is that the timer service knows the topology of the server farm, and you can load balance the jobs across all the servers in the farm or tie them to specific servers that run particular services.
Some changes have been made in Windows SharePoint Services 3.0 to use these services in easier way. First, farm administrators can now see all the registered and timer jobs in a server farm in Central Administration. To do this, on the Operations page, under the Global Configuration section, select Timer Jobs Definitions. In addition to the registered jobs, the Timer Job Status page contains a list of all the jobs and the status and outcome of the last execution of each job.
For creating timer jobs by we create a it by using a single class that inherits from the Microsoft.SharePoint.Administration.SPJobDefinition class. We must deploy the assembly that contains this class to the global assembly cache (GAC). Then we must deploy, or install, the timer job into the server farm.