System administrator

A system administrator describes someone responsible for running, or running some aspect of, a computer system.

The precise meaning varies. Organisations with very large or complex computer systems typically divide up computer staff according to specialism, in which case a system administrator ("sysadmin") is someone responsible for the maintenance of the existing computer system, and who is specifically not :

Technical Support
DBA
Application Analyst
Programmer
Network Specialist
System Architect

In smaller organisations, not all of these specialisms exist separately, and the term "system administrator" is used in a much more generic way - they are the people who know how the computer system works and can respond when something fails.

A system administrator's work will almost always include:

performing backups
applying any operating system updates, and configuration changes
installing and configuring any new hardware/software
adding/deleting/modifying user account information, resetting passwords, etc.
answering technical queries
responsibility for security
responsibility for documenting the configuration of the system.
troubleshooting any reported problems.

But may (particularly in smaller organisations) include much of the job description given in the separate articles listed above.