Lua is a lightweight, embeddable scripting language. It's garnered a lot of popularity partly due to it's use in many popular games. A good example of this is World of Warcraft which uses an embedded version of lua to drive the behavior of the UI elements in the game.
A good intro to the language can be found here: http://computer-programming-languages.suite101.com/article.cfm/a_brief_introduction_to_lua_scripting
And the official online reference for the language can be found here: http://www.lua.org/manual/5.1/
