I am not familiar with raid system.

Could someone tell me if I got three 18GB drives with raid 5 configuration. How much usable space do I get??

Thanks in advance...

Can anyone explain in detail.

You've got roughly 36GB usable space with RAID 5 and 18GB drives. Look at http://www.raid.com/04_01_00.html for a decent overview of various RAID levels and what they do.

Dan