RAID allows you to set up a team of Disk Drives to act as one. There are configurations that provide additional capacity, additional speed, and additional redundancy, as well as combinations. Many common configurations do not requires any special Hardware.
RAID optimized for speed gets additional speed by overlapping the read of the next block with the data transfer from the previous block. This only provides an improvement in speed when the larger operation is not interrupted by ANY stray reads or writes at other areas of the Disk. Setting up a single RAID for reading and writing data at the same time does NOT provide a measurable speedup.
Some versions of RAID (e.g., RAID 5) calculate and store check blocks that can provide some redundancy across three or more drives. If one drive fails, the remaining data and the check blocks can be combined to recreate the missing data. This requires that check blocks be calculated and checked as data go by, which requires additional Hardware assistance to be adequately speedy.
The Apple RAID card is showing signs of advanced age, and has not been updated in quite a while. Many here consider it a poor investment at this time. It limits you to 2.2TB per drive. Its battery has been a source of major problems.
You can use Disk Utility to create Mirrored RAID, Striped RAID, and JBOD RAID without a need for any card.Mac Pro (Early 2009), Mac OS X (10.6.8), & Server, PPC, & AppleTalk Printers