RAID is a method of setting up multiple hard drives so that data is not lost should one of them fail. They can also provide much faster response times than a single disk, and so are often used in situations where HDD access speed is a factor such as Servers or high end workstations, especially when doing multimedia work.
A RAID array can be setup using any disk technology, but will need a RAID controller in order to manage them. At the cheapest end this may be managed by the Operating System, but it is far better to use a dedicated hardware RAID Controller. This will present the array as a single disk to the OS.
click here has some pretty good explanations.