There are many levels of fault tolerance, the most basic being the ability to continue operation in the event of a power failure or destruction of data. These include:

RAID Drives ( Redundant Array of Independent Disks):
Two or more inexpensive hard drives where data is constantly being backed up using one of a variety of different methods, including Data Striping and
Disk Mirroring.

Tape Backup: Regular Backups of your network data saved onto a one or more removable tapes.

UPS (Uninterruptable Power Supply): Plug your computer into the UPS and it's battery will keep you going in case of a power failure.

SAN (Storage Area Network) and NAS (network-attached storage): Two different solutions that give you a secure, scalable place to store your data separate from your network server. If the server goes down, your data remains.

Redundant Hardware: Doubling of power supplies, hubs, hard drives, etc.