Raid or Redundant Array of Independent Disks is a method of increasing redundancy of your hard drives. one of the drives can fail without losing your data. Depending on the controller, it can be faster than a single disk or slower. Raid 6 is now becoming popular, it allows for 2 disk failures. You give up storage capacity for redundancy.
My general advice is that you should not use RAID unless your goal is to increase performance or capacity beyond the limits of a single drive. The purpose of the redundancy in RAID is not to improve reliability, but rather to help ameliorate the colossal loss of reliability that would otherwise occur in a multi-drive set.
There are two main problems with using RAID:
1. You usually build RAID arrays using hard drives that are substantially identical, because otherwise you take a significant performance hit. The odds, therefore, are good that any mechanical flaw in one will also be present in all the others. Now bear in mind that most of the operations performed on one disk are also being performed on all the others—the same seeks, the same head parking, the same number of powered-on hours, etc.—which means that a staggering percentage of those mechanical flaws tend to show up at about the same time. When one disk fails, the odds of a second disk failing before you can clone the first one is pretty high, and if the second disks fails during that interval, the odds of a third disk failing before you can clone either of the first two approaches 100%.
2. The mean time between failures (MTBF) for a set of n drives is equal to the MTBF of a single drive divided by n. So if you have five drives, you are five times as likely to experience a failure as if you have only a single drive, assuming your failures are, in fact, random rather than being caused by a design flaw.
The additional parity disks compensate for #2 somewhat, though not completely. They do nothing to help with #1. And that's not even factoring the added risk of catastrophic data loss caused by bugs in RAID controller firmware. When you factor that in, your need for backups is likely to be significantly greater for a RAID array than for a single drive of equivalent size (assuming such a drive exists).
Also, although it might go without saying, it is still probably worth pointing out that even under ideal circumstances, RAID is not a substitute for proper backups. That file you overwrote is still gone whether you're using a RAID volume or not.
In short, unless a single hard drive can't keep up with your throughput needs (e.g. if you're working with uncompressed 4K video) or you cannot buy a hard drive big enough to meet your needs, you're almost always better off not using RAID, and instead using an additional hard drive as a true backup. With that said, if you absolutely have to have a single volume with larger capacity or faster performance, RAID is a good solution.