Lua is a scripting language developped at the University of Rio de Janeiro (PUC-Rio). From the homepage (www.lua.org):
Lua is a powerful, fast, light-weight, embeddable scripting language.
Lua combines simple procedural syntax with powerful data description constructs based on associative arrays and extensible semantics. Lua is dynamically typed, runs by interpreting bytecode for a register-based virtual machine, and has automatic memory management with incremental garbage collection, making it ideal for configuration, scripting, and rapid prototyping.
Included here is a quick reference of the basic Lua commands from Lua 5.1.4 and also a reference of all additions made to Lua for use with Core4.
More information about Lua can be found at www.lua.org.
Also, there are two official books:
Lua 5.1 Reference Manual
by R. Ierusalimschy, L. H. de Figueiredo, W. Celes
Lua.org, August 2006
ISBN 85-903798-3-3
Programming in Lua (second edition)
by Roberto Ierusalimschy
Lua.org, March 2006
ISBN 85-903798-2-5