What is a dive computer? How does it work?

Over the years, scuba diving has become more technical as seen in dive computers.  These small digital devices take a lot of guesswork out of a dive such as monitoring depth, absorption of gases, decompression stop times, length of dive, and more.  The purpose of the dive computer is to provide divers with a more enjoyable but also safer underwater experience.  Instead of manually planning a dive, the dive computer would provide exact information during the dive based on software containing specific calculations.