It is, but here's a few (very basic) basics:
RAID-0 - involves using the disks to their fullest capacity. There is NO redundancy with this (i.e. if one disk dies, you lose everything!)
RAID-1 - mirroring - data is written to two hard drives simultaneously. Available capacity is only the size of one of the disks, obviously, because the other one is a real-time backup, in effect.
RAID-5 - "striping with parity" - a grand term for a pretty grand design. RAID-5 stores the data across 3 (or more) disks. The data is written to all but one of the disks in the array, and a parity check (very simple error checking) is stored on the third disk. This block of data + the parity block is called a "stripe", hence the term "striping with parity". Available capacity = total size of all but one of the disks in the array.
So say you had 4 x 100 GB disks in a RAID 5 array, you would get 300GB total storage, which is pretty efficient.
Redundancy-wise, RAID 5 allows one disk to fail whilst still preserving all the data and being fully functional. Obviously if a second disk should subsequently fail before replacing the first one, then you're pretty screwed!