(Or utility program, tool) Any software that performs some specific task that is secondary to the main purpose of using the computer (the latter would be called application programs) but is not essential to the operation of the computer (system software).
Many utilities could be considered as part of the system software, which can in turn be considered part of the operating system.
The following are some broad categories of utility software, specific types and examples.
disk checker: fsck
* Files and directories
copy, move, remove: cp, mv, rm, xcopy
format conversion: atob
antivirus software: avast, Norton Antivirus
firewall: Zone Alarm, Windows firewall
* Editors for general-purpose formats (as opposed to specific formats like a word processing document)
file transfer: ftp, rcp, Firefox
file synchronisation: unison, briefcase
directory services: bind, nslookup, whois
network diagnosis: ping, traceroute
* Software development
installation: apt-get, msiexec, patch