In previous versions of SharePoint (or other platforms), if you had some task you wanted to perform on a scheduled basis, you'd have to either create a console EXE and schedule it to run via Windows Task Scheduler (ala AT.EXE) or create a Windows Service that went to sleep for a period of time. In order to install (and maintain) these tasks, you had to have console access to your production SharePoint (or other app) servers... something IT or admins wouldn't easily hand out.Addressing this issue, Microsoft has added something called timer jobs to Microsoft Office SharePoint Server (MOSS) 2007. Microsoft uses timer jobs to do things like dead web cleanup (purging unused sites from site collections) among others. To see what other timer jobs are out there, from Central Administration, click Operations and then Timer Job Definitions. Not only does Microsoft use timer jobs in MOSS, but you can create your own custom timer jobs to do your own scheduled tasks. What's nice about this is once your timer job has been installed (something you can easily do with a solution & a feature), you can view it's status through Central Administration and even disable/enable it... all without console access to your production servers! Another cool thing is that when your job runs, MOSS passes it the GUID of the content database for the site the job is registered with. You can use this GUID to obtain a reference to a content database, then a site collection, and finally a site within the collection (SPWeb).
How do you build one? Well, unfortunately the documentation is lacking here... there isn't a single article in the SDK talking about creating custom timer jobs and the necessary objects aren't well documented either.
MVP Andrew Connel has described the Custom Timer Jobs on SharePoint well in depth on his blog http://www.andrewconnell.com/blog/articles/CreatingCustomSharePointTimerJobs.aspx. Awsome functionality. Thanks Andrew.