"The standard RAID levels are a basic set of RAID configurations that employ the techniques of striping, mirroring, or parity to create large reliable data stores from general purpose computer hard disk drives. The most common types today are RAID 0 (striping), RAID 1 and variants (mirroring), RAID 5 (distributed parity) and RAID 6 (dual parity). RAID levels and their associated data formats are standardized by the Storage Networking Industry Association in the Common RAID Disk Drive Format (DDF) standard."
Jose Koshy SamuelCISSP, CISM, CISA, CRISC, MBCI_Linked7CxTwR
Most SAN or NAS boxes have some raid levels built into them. RAID is used to provide levels of reliability in the event of disk failure. Various RAID levels are explained here http://www.thegeekstuff.com/2010/08/raid-levels-tutorial/ RIAD 5 is standard; you also need to keep in mind the time taken to rebuild the data which is different for different RAID configurations. The primary consideration should be the RTO & RPO set by your business, how much downtime can you affrod?