What are mods?
Mods are content that is added into the game or content that is already within the game, but changed by Modders.
Mods can range from game-play tweaks like upping or lowering stats on something to complete overhauls of systems or additions of new races, classes, abilities, maps and more.
Where can I find mods?
Here is the most known site (that I know of at least) for uploading mods to certain games. http://www.nexusmods.com/divinityoriginalsin2/?
How do I install mods?
Each mod has it's own way of being installed into your game, some are extremely simple drag and drops and others can become far more complicated, however the page containing the mod will (almost) always have instructions on installation located on them.
Mods List Edit
A few things first. 1) Currently the game is still in backer alpha/beta and thus mod production is (near) non-existant. 2) Not all mods produced will be listed here, but do feel free to list some if you come across some interesting ones. 3) It'd be nice if the list could contain appropriate mods, I'm sure many don't wanna see "Now you can make your whole party butt naked!" mods all over the place and if they do they can seek them out themselves.
Bardvark's Bard mod.
Adds a new class, the Bard and tweaks some mechanics.