When you're building a server, you're usually creating a platform for services that you deem important - important enough to warrant the effort and the dedication of server hardware (regardless of whether the hardware is actually server class hardware). The functioning of important services should not be impeded by such a mundane thing as a hard disk failure - and yet the hard disk is one of the most vulnerable parts of the computers of this day and age.
Fortunately, Linux can help us out here, as it does in many other server-related area's. Out of the box, it supports software RAID, which can be used to bolster server reliability. Many are the places on the Internet where you can find out more about it, so we limit ourselves to the following, summary explanation.