Hi newcomer and welcome to the Tom's hardware forum.
RAID 1: Called mirroring and you need at least two HD, is for save the same data in two HD at the same time, you PC can work even if one disk fail, but not the two. This only increase the "security" of your files, not the performance of your rig.
RAID 5: You need at least 3 HD, and you get "security" for your files and increase the performance of your rig. Onl one HD can faild and you need replace it as soon as possible because the rig lose performance without the other HD.