RAID configurations come in a few different varieties, each with its own pros and cons. Some provide increased speed while others provide fault tolerance. Let's focus on fault tolerance and how many drives can fail in each configuration without harming your system.
RAID 0: This configuration is built for speed and allows you to use all of your available space. RAID 0 provides no fault tolerance. Any drive failures will cause data loss, so do not use this on a mission critical server.
RAID 1: A RAID 1 configuration is best used for situations where capacity isn't a requirement but data protection is. This set up mirrors two disks so you can have 1 drive fail and still be able to recover your data.
RAID 5: This RAID configuration provides the best value for the money. RAID 5 gives you access to more disk space and high read speeds. Write speed suffers a bit in this set up but you can withstand a single drive failure and be ok.
RAID 6: Because of parity, RAID 6 can withstand two disk failures at one time. This can be simultaneous failures or during a rebuild another drive can fail and the system will still be operational.
RAID 10: This RAID can survive a single drive failure per array. It is a very fast setup with redundancy built in and requires a minimum of 4 drives to be operational.