RAID1 is the simplest system to configure and only requires two disks the second of which simply mirrors the first. If one disk fails the same information is stored on the second disk thereby preventing any data loss. The disadvantage with RAID1 is that storage capacity is effectively halved.
RAID5 on the other hand offers more flexibility in that multiple disks are used so the storage capacity can be massive. RAID5 is also highly secure as data is striped, or saved, across all the disks in the array with parity blocks created on each disk.
This means that in the event of a disk failing it is simply replaced, or hot swapped, with another disk. The data is then rebuilt using the parity blocks from the other disks in the array.
Once you have a NAS enclosure in place you need to fill the bays with hard drives. These drives, by leading brands HGST, Seagate and, of course, Western Digital are specifically manufactured for NAS enclosures as they need to have a large capacity, robust reliability and fast data transfer speeds.
Many NAS drives will be cross compatible with other manufacturers enclosures which means the best value drives can be purchased irrespective of the brand (though it is always advisable to check compatibility).