The following is a brief description of 2 of the main types of disk configuration you will deal with; RAID 5 and RAID 10. A notable differences is performance. In both cases disks are striped which at a high level means spreading out parts of every file across multiple disks. This provides faster reads and writes.
RAID 10 is more expensive than RAID 5 because a minimum of 4 disks is required (2 striped, 2 mirrored). The striping provides the performance and the mirrored disks look after redundancy.
With RAID 5 a minimum of 3 disks are required; 2 striped and a third disk to store a Parity Bit which acts like a CHECKSUM. The Parity Bit is RAID 5s’ equivalent of redundancy management.
Regarding SQL Server performance, gains can be found with both reads and writes. However the big gain with RAID 10 is with disk writes. RAID 10 doesn’t have to calculate a Parity Bit and then write it to the third disk like RAID 5 does. As a result, writes can be almost twice as fast with RAID 10 compared to RAID 5. Bill Graziano of www.sqlteam.com provides a great diagram to illustrate the point : http://weblogs.sqlteam.com/billg/archive/2007/06/18/RAID-10-vs.-RAID-5-Performance.aspx. This diagram is based on OLTP simulated transactions using SQLIO.
Of course it depends on the application running on the SQL Server, and the level of reads vs. writes. But when it comes to SQL Server, I’d always recommend RAID 10 over RAID 5.