TDLib or Three's Derma Lib is a derma library created by Threebow allowing you to create beautiful Garry's Mod UI quickly and without hassle. No more messing with paint functions.
View the documentation here - GitBooks isn't letting me make any more changes to those docs or even use my account, so please keep this information in mind when you are using classes - it won't work without it!
Move the file into the lua/autorun/client of your addon. Do not edit the file in case there are multiple copies of it on the server from other addons, to avoid confliction. If you feel a modification is neccesary, open a pull request.