The greatest secret of the Glorious PC Master Race is that – shh - most games on PC and games on consoles are basically the same, whether you're herding it along with a keyboard or a games controller. But the PC modding communities for games like Skyrim are the exception to that rule: talented individuals (or teams) working long hours to create all manner of extra content and gameplay tweaks, while console players must wait like Dickens characters for their next serving of official DLC. The PC is an immeasurably better place to play Skyrim than on console, and its modding community is why.
The jump-off for Skyrim's legions of modders is the Creation Kit - a suite of world-building tools that let players and would-be designers play around with the game's toybox of items, characters and dungeons. The Creation Kit is free to download (as are its equivalents for other Bethesda games) and very powerful: depending on your skill level you can create anything from simple dungeons to fully-voiced, ten-hour questlines with new animations, weapons, spells and lighting effects. Or, if you're Scott Caunce, you can cover everything in palm trees.