Function Dump
This is the location for the listing of all of the Lua based functions, ones that are built into the system. These are not to be confused with defined functions, ones that are made in your own code. These are changed/added to by the ROBLOX staff for the base Lua code, and cannot be overwritten or edited.
These are broken up into 6 groups, where you can find more information about what you want to do on each page.
Core Functions
These are what help the Lua code run on Roblox. In here you can find listings for file manipulation, metatables, error handling and other basic system functions. If you aren't sure where the function is listed, look here first.
Coroutine Manipulation
Here is the listing for coroutine manipulation, which allows you to set several things working at once and for a limited time of running. Take a look here and here for more detailed explanations of coroutines.
String Manipulation
These functions let you do fancy things with words, such as changing the order of a string, comparing two strings and using the chat commands to do all kinds of fun stuff.
Table Manipulation
This section tells you how to play around with long lists of things and do fun stuff as well, such as reordering a table, sorting them, and searching for a specific item in a table.
Mathematical Functions
Just as the name implies, this is a listing of all the math functions in Roblox Lua. Here you can find a listing of everything from trigonometry to a value so huge it doesn't have a real number.