System Administration - Hardware is the physical things in a computer that you can touch e.g. keyboard, mouse, touchscreen, speaker, hard disk, memory chips, Processor etc.
Software is the set of instructions that the computer runs.
A person employed as a Systems Administrator is responsible for the upkeep,  configuration, and reliable operation of software. He/she will sometimes replace or upgrade computer hardware.

Two of the most important roles a System Administrator provides are:
1.    Backups
2.    Security

Useful link for more information:
http://en.wikipedia.org/wiki/System_administrator