Since some fellow kernel hackers decided to implement RAID for the Linux kernel, this technique can be used by low-end PCs, and regular people can be satisfied by the improved performance and data security. The stored data is distributed over all disks in a well-defined way. The data is stored in a redundant way over the disks, so in case of failure, data is recoverable. The reason for this is due to ability to request data from the disks in a parallel fashion. The easiest way to do this is called striping mode or RAID level 0, but it doesn't contain any redundancy.
Redundancy is achieved in different ways.
The simplest is to store the data on two equal disks. This is defined in RAID-1, also known as mirroring. Of course, one gets performance increase only when at least four disks are used. More efficient redundancy is obtained when instead of duplicating all the data, a unique checksum is generated and stored with regular data. If a single disk should fail, one is able to reconstruct its data by using all data chunks of that stripe together with the calculated checksum.