Have you tried the search button? This have been explained by Macca (I think) about 5 times or so.
Anyway, here it goes:
Gamelogics are just like any other troop, it's just invisible, can't carry weapons, moves from point A to point B in 0 seconds, can't be player controlled and have 0 AI, thus it makes NOTHING unless it's told to do so.
BIS used them as conversation makers. You see, a game logic moves from A to B extremly fast. So if you have a bunch of WPs with delayers on them, you can make a conversation with it, when GL move to A, X says "Hello", move to B, Y says "Hello, go screw yourself". Get it?
Players and mission editers use them mostly as markers. For example, you need to move a unit without him going anywhere(setpos command), you put out a GL where you want him and type this:
loon1 setpos getpos GL1
Hope it helped a bit.
:beat: *Gets Shot* :beat: