Lua is a powerful, efficient, lightweight, embeddable scripting language. It supports procedural programming, object-oriented programming, functional programming, data-driven programming, and data description.
I actually came upon it when I discovered it is used extensively behind the more complex functioning of watch faces designed in WatchMaker - see http://watchmaker.haz.wiki/lua.
Lua is currently the leading scripting language in games. Lua has a solid reference manual and there are several books about it. Several versions of Lua have been released and used in real applications since its creation in 1993. Lua featured in HOPL III, the Third ACM SIGPLAN History of Programming Languages Conference, in June 2007. Lua won the Front Line Award 2011 from the Game Developers Magazine.
More info on Lua itself at http://www.lua.org/about.html