Have you always dreamed of replacing Skyrim's hero with Dora the Explorer? Of seeing Nicolas Cage everywhere? No problem—the Mod is here for you. A large number of bright minds enjoy tinkering with games to help you realize your wildest dreams.
There's indeed one genre of player that stands out from the others: the PC gamer. The latter is above the futile console wars, for a reason: he is superior to all of that. The reason is simple: while console players content themselves with playing the games publishers offer them, PC players dig into the game's files to make modifications.
These modifications, called Mods within the "PC Master Race" community, are as varied as they are numerous, somewhat like a homemade DLC!
Let's take the example of Dark Souls 1, the legendary FromSoftware game. The most downloaded mod is called "DS Fix" (Dark Souls Fix); it makes the game run more smoothly and look nicer and fixes certain visual bugs in the game. While the mod "Estus Quest" deprives you of your - very precious - health flask until you have defeated one of the final bosses of the game. If you have never played Dark Souls, know that the slightest basic enemy will take away three-quarters of your health bar. So that alone seriously complicates the game.
That is for mods called "useful"; other games have mods, just very funny. Thus, a Skyrim mod lets you replace dragons with "Thomas the Tank Engine". In Left 4 Dead, a mod offers to put a filter on your flashlight to make appear... Nicolas Cage's head...
Now that I’ve presented mods to you, a question should nag at you: "But what is a mod for?"
Nothing. More seriously, mods are more like technical feats than anything else. It's more an opportunity for their creators (master programmers), known as modders, to take on a challenge. It's also a way for future developers to get their hands dirty on an existing game before jumping into the big bath and creating their own game from scratch.
The big plus of mods compared to downloadable content (what we call DLC) is that they are entirely free, much to the dismay of Microsoft who tries to commercialize them.
But mods, they're like alcohol. They should be consumed in moderation. If you install too many, your game will lag, or worse, it won't even launch.
It's up to you to make the right choice.