From LQWiki

fsck (file system consistency check) is a command used to check filesystem for consistency errors and repair them on Linux filesystems. This tool is important for maintaining data integrity so should be run regularly, especially after an unforeseen reboot (crash, power-outage).

Usage:

fsck [-sACVRTNP] [-t fs-optlist] [filesystem] [fs-specific-options]

Filesystem can be either a device's name (e.g. /dev/hda) or its mount point.

fsck run with no options will check all devices in /etc/fstab

It might be neccesary to run fsck from single-user mode if you need to check and restore a possibly damaged filesystem, especially is /usr is to be checked.

fsck is merely a front-end to other more specific file-checkers which are located in /sbin, /etc/fs an /etc first followed by the $PATH directories.