I've searched on the wiki but i cannot find it, here is my last resource.
A quick wiki search will bring you to this page: http://wiki.roblox.com/index.php?title=Global_function
A _G command essentially holds a variable or a function that can be used in other scripts (_G meaning global).
So if I went_G.Cats = "Cats"
that would save the word cats in a global variable.
print(_G.Cats) -- "Cats"
Also just be aware that Global values saved in a localscript will not be available to server scripts and vice versa.