RAID 1 (or Redundant Array of Independent Disks, level 1) is one of many different schemes of mirroring and striping computer data drives in order to ensure that a minimal amount of data is lost. The other RAID levels are RAID 0, RAID 2, RAID 3, RAID 4, RAID 5, RAID 6, and RAID 10; each level has a different degree of tradeoff between cost and performance.
RAID 1 merely refers to straightforward disk mirroring. In other words, each drive has a "mirror" drive which is an exact duplicate of its contents. This is so that if one of the disks fails, there is an exact duplicate of the disk already full of data and ready for use, thus providing some insurance that your system won't die in the case of a disk failure.
The advantages and disadvantages of RAID level 1 are quite clear. It has identical performance to normal disk usage in daily use. It also guarantees quick recovery from disk failure, because all that you have to do to recover from it is simply copy the mirrored drive onto a new one. The cost is slightly prohibitive, as the price for storage is exactly doubled; each drive must have an exact duplicate on the system.
One important consideration in a RAID 1 setup, and perhaps even more importantly in other RAID disk setups, is the idea of homogenity among the various drives. If all of the drives have identical sizes and setups, then the drives can be moved around quickly and efficiently. This also means that the possibility of having a single hot spare as a backup to the RAID 1 setup is quite feasible; a hot spare is a ready-to-go drive without any data that can be plugged in quickly and immediately be integrated into the system. Without homogenity, having hot spares available becomes considerably more expensive and logistically challenging.
Another important note is that RAID 1 does not replace the concept of backups of critical data. Having a drive mirror is great from a reliability standpoint, but it is not a backup and should not be treated or relied upon as such. Even with RAID 1 set up with drives, backing up data regularly is still vital for the simple reason that if both drives were to be damaged (a natural disaster or extreme bad luck), the data still needs to survive, especially if you are in a situation where RAID is important.
RAID 1 is generally popular for applications that require a high degree of reliability with fast recovery, with cost and speed of access not being major concerns. An example of a good RAID 1 application is patient record storage at a doctor's office, in which having the data always available and the ability to recover from failures fast is quite valuable. The database itself won't be immense, meaning that the cost of double drives wouldn't be prohibitive, and the actual access speed, in terms of seconds, isn't a vital issue, either.