RAID level 1 - mirror (just drives) or duplexing (duplicate controllers).
  SCSI or ATA-IDE

  Usually just 2 disks or multiple pairs (combination of RAID 0 and 1).

  Drives should be identical.

  All data on first disk mirrored on second disk.

  This provides error protection.
    Minimal load on CPU or controller for processing data.
   
  Better designs support parallel reads of a file and reading of different
    files at the same time on multitasking systems.

  Writes slightly slower than single drive because data written to both disks.
    Spindles not synchronized, and write not done until both completed.
    When used with newer drives with 32/64MB caches, even writes can 
      equal non-RAID drives.

  Mirroring can be hardware or software(OS) implemented.
    If OS, adds load to system, so slower drive and system speeds.     

  Failed disk is simply replaced and data from other copied over.
    Often RAID uses "hot" swappable disks - changeable while running(SCSI, SATA).
 
  Double the cost for drives over single drive.

  Recovery hardware cheaper than other RAIDs because of simplicity.

  Modern RAID 1 systems use default clusters for "stripes".
    More often used to access different files concurrently on different drives.

  Excellent data protection.
  Good to Excellent read throughput.
  Acceptable to Very good write throughput (latest drive technology).
  Drive cost 2X
  But controller simpler than other RAIDS.
  Recovery simple and reasonably quick (just copy the good drive).