LÖVE is an unquestionably awesome 2D game framework, which allows rapid game development and prototyping in Lua.
This tutorials explains how to use the global callback functions in LÖVE.
Posted by thelinx on Mar 13th, 2009
Basic Client Side Coding.
This tutorial is taken straight from the Wiki.
The callback functions in LÖVE are used by the engine to perform various tasks and are all optional. However, a fully-featured game experience would probably utilize nearly all of them, so it's wise to know what they are.
This function gets called only once, when the game is started, and is usually where you would load resources, initialize variables and set specific settings. All those things can be done anywhere else as well, but doing them here means that they are done once only, saving a lot of system resources.
This function is called continuously and will probably be where most of your math is done. 'dt' stands for "delta time" and is the amount of seconds since the last time this function was called (which is usually a small value like 0.0025714).
love.draw is where all the drawing happens (if that wasn't obvious enough already) and if you call any of the love.graphics.draw outside of this function then it's not going to have any effect. This function is also called continuously so keep in mind that if you change the font/color/mode/etc at the end of the function then it will have a effect on things at the beginning of the function. For example:
This function is called whenever a mouse button is pressed and it receives the button and the coordinates of where it was pressed. The button can be any of the constants. This function goes very well along with love.mousereleased.
This function is called whenever a mouse button is released and it receives the button and the coordinates of where it was released. You can have this function together with [[love.mousepressed]] or separate, they aren't connected in any way.
This function is called whenever a keyboard key is released and receives the key that was released. You can have this function together with love.keypressed or separate, they aren't connected in any way.
Those are the callback functions and their basic usage.
You can view more tutorials like this at the Wiki!