The creation of modifications or "modding", is the process of altering or adding to a computer game for the purposes of providing a different and/or improved experience.
Bethesda confirmed mod support under the new Creation Engine in January 2011, saying, "We've always been impressed with what the community has done with our tools. Like the Elder Scrolls Construction Set for Morrowind and Oblivion, we plan to release the Creation Kit so you guys can mod Skyrim."
This applies only to the PC version for now, though the studio is investigating bringing it to consoles in some form post-release. The likelihood of this actually happening is anyone's guess, however.