Ruby is a dynamic, reflective, general purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was initially developed and designed by Yukihiro "Matz" Matsumoto. It is based on Perl, Smalltalk, Eiffel, Ada, and Lisp.
Ruby was conceived on February 24, 1993 by Yukihiro Matsumoto who wished to create a new language that balanced functional programming with imperative programming. He stresses that systems design needs to emphasize human, rather than computer, needs. Here is quote from wikipedia on Ruby
Often people, especially computer engineers, focus on the machines. They think, "By doing this, the machine will run faster. By doing this, the machine will run more effectively. By doing this, the machine will something something something." They are focusing on machines. But in fact we need to focus on humans, on how humans care about doing programming or operating the application of the machines. We are the masters. They are the slaves.
Ruby has two main implementations: The official Ruby interpreter often referred to as the Matz's Ruby Interpreter or MRI, which is the most widely used, and JRuby, a Java-based implementation that runs on the Java Virtual Machine.
So why Ruby?
Increasing popularity of Ruby with some very popular projects including Watir (a popular testing tool), Redmine (a comprehensive project management application), and many others. developed using Ruby. Besides there is the increasingly preferred web application development framework called Ruby on Rails.
Best place to find out more about Ruby is from Ruby-lang.org.
Basic programming language fundamentals and beginner level understanding of any programming language.