Root privileges

The root user (uid 0) is the only user that can perform certain actions:

Read/write/chown/chmod any file, regardless of permissions/owner
Bind low (<1024) network ports
To provide inbound services
To create outbound services that should be 'trusted'.
Access raw network devices to create custom packets (nmap), create ICMP packets (ping), change interface parameters (IP address)
Become another user/group
Use advanced filesystem attributes
Reboot machine
Increase process priority (renice --20 -p PID)
Send signals to any process