Resolve startup issues and perform disk maintenance with Disk Utility and fsck

If your computer won’t start up normally, you may need to use a disk repair utility to fix the issue. Mac OS X includes two utilities for this—Disk Utility and fsck. You can also use these even when your computer starts just fine but you want to check the disk for possible file system issues…

A decent Apple Knowledge Base article which covers repairing your start up disk using either “fsck” or Disk Utility. Tiger users should note that Apple recommends using Disk Utility whenever possible instead of the “fsck” (Unix “file system check”) command for performing repairs and/or maintenance.