When implementing a RAID array, you can do so with special hardware intended for that purpose. Many OSes nowadays, from Windows to Linux, include software RAID that doesn't require anything beyond the disk controller on your motherboard.
There are some advantages to hardware RAID. In many software RAID setups, such as the Linux implementation, you have to be careful to ensure the system will boot off of either drive in case of a failure. The BIOS provided by hardware cards normally takes care of this for you. Also, in cases where a failed drive has been replaced, it's usually easier to set up an automatic rebuild with a hardware RAID card.
When hard drives fail, they can take down the entire system in the process ...
