Mods as mentioned can more or less be for anything concerning the game, from new content to enhancing either graphics or game-play, frame rates etc..
However, as with a game like Arma 2 for instance where there are literally 100’s of mods/islands, you can and will get bogged down by which ones to install.
So play the game of your choice and see whilst your playing what you would like to see in the game that makes it more fun, different or just plain better, then look to see if someone in the community that surrounds that game has made a mod that may cover what you want.
That would be the best way, chances are someone will have...
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