Plugins for KSP allow you to add new features and change the behaviour of the game.

The primary plugin site is http://kerbalspaceport.com. However I found more up to date plugins and mods available on the Kerbal Space Program forums, http://forum.kerbalspaceprogram.com/.

The first plugin I have used is the "Visual Enhancements" mod, which primarily adds clouds, making for a much more dynamic launch experience:

I'm not sure if it also changed the mountain terrain, or if I just happened to land in a nice area: