Broadly speaking, security comes in two forms:
1. Physical security. This includes actual security personnel guarding the facilities as well as surveillance equipment and password protected entry points.
2. IT security. This includes firewalls and other software implemented to thwart virus attacks and other malicious software intrusions into the system.
Here's an interesting link with more details on data center security:
http://www.processor.com/editorial/a...0p11/20p11.asp