Oversimplifying - In RAID 0, all your hard drives are clubbed to make it look like a large single hard drive. Each piece of data is written partially on each hard drive inside. So if any 1 hard drive fails, you lose all your data. RAID 5 writes the data in such a way that 1 hard drive is always redudant. In case any 1 hard drive fails, you still have access to your data. RAID 6 writes the data in such a way that 2 hard drives are redundant. Your data is not lost, even if upto 6 hard drives are failed.