USA Trucker,

I make no confusion between modding a game or having an expansion.

1) Expansion is made by the company who issued the game or a company who bought the license of a game.
2) Mods are made by users and are free.

Sometimes expansion are made from the source of a mod.

Mod can totally modify the game play of a game and change all the rules of the game.

FYI ==> Check Warcraft III modding, CounterStrike Source modding and so on.

Example they did a First Person Shooter from Warcraft III, a tower defense ....