X.Org (www.x.org) is the X server that is used with Ubuntu. The base distribution consists of many packages including the server, support and development libraries, fonts, various clients, and documentation. An additional 1,000 or more X clients, fonts, and documentation are also available in the Ubuntu repositories.
/usr directory and its subdirectories contain the majority of the
Xorg software (along with a lot of other stuff; the location is not exclusive to X). Some important subdirectories are as follows:
/usr/bin—This is the location of the X server and various X clients. (Note that not all X clients require active X sessions.)
/usr/include—This is the path to the files necessary for developing X clients and graphics such ...