VOID - When!?

Hello - I just don’t get the difference when to use VOID or Not …
with so many tutorials I have read, some use void, and sometimes they don’t within very similar circumstances …

function onEnterFrame(Void):Void
{
}
//and
onEnterframe = function():Void
{

}
//and there's even a lower case 'void' too .. >>
//I usually just use 
onEnterFrame = function()
{
}

Can someone explain when and not to use it - or should it always be used!?
thanks …