raid cards create raid volumes
they are 3 types of raid that are used most common
raid 0
two cards share 1 volume to increase speed
 
raid 1
1 volume identical copies for redundancey
 
raid 5
multiple drives ( 4-5 ) it means any drive can fail  ( or two in case of 5) so that it is very redundanct
 
 
motherboards dating back to 2006 or newer wil have raid build in
 
the diffrence is what type of raid , what controller , and how fast
cheap motherboard ussualy have only raid 1 or 0 or not at all.whereas motherboards 80+ ussualy have all raid support build in by controller (AMD or intel) some drives can raid support on secondary controller , incase all your sata ports are filled in, in my exprince they dont perform as well as the intel/AMD one.
 
 
 
only advantage by gettign raid card is only if you speed 150+ on one that has build in cache and battery back up so it speeds up and in case power failure it  can continue to do the work. they are advnatage if you have raid 5 or 6 or 10 where the cpu needs to constancey think about where to put data where as the raid card will speed up the process by factor of 10 - 50. for raid 0 there is no such improovement.
 
 
 
there is no reason to buy raid card if you mobo is 3 years or newer  unless you are occupying all your sata drives . you may find yourself getting slight improvement but not worth the price ( im talking about 10-50mbs ) around 1% improvement with intel 520 drives... for cost of an entire drive.