There is many forms, hmm, most common 2 are; RAID0, this is 2 Hd's running as 1 Hd, so 2 150gb becomes 1 300gb, Data is written to both Hd in Stripes of say 250kb, so in effect when you read/write data its faster because you are retrieving it from 2 drives.
RAID1 is also known as mirroring, if you have 2 150gb drives this =300gb of storage. If you use RAID1 you will have 150gb of storage and the other 150gb is a back-up in case your Hd fails and you lose all your data, because all the info you store is written to both Hd's in effect creating a mirror of your data.
lol just the tip of the iceberg