X server

The X.Org server, part of the X.Org releases, is the main component of the X Window system, which abstracts the hardware and provides the foundation for most graphical user interfaces, like Desktop environments or Window manager, and its applications.

global USE flag
Portage knows the global USE flag X for enabling support for X in other packages (default in all desktop profiles):

xorg-drivers
is a meta package to pull in the wanted drivers: Follow the links for information how to set up these devices.
 * Graphics adapters
 * Input devices

mesa
(see Wikipedia) is a graphic library that provides a generic OpenGL implementation.

xorg-server
Now install.

Alternatively you can install the meta package, which installs the full X.Org release with additional fonts and utilities. They're not necessary for a working desktop.

xorg.conf
The X server is designed to work out-of-the-box, with no need to manually edit Xorg's configuration files. It should detect and configure devices such as displays, keyboards, and mice.

However, the main configuration file of the X server is the xorg.conf.

Boot service
Usually the X server is started by starting a display manager automatically on boot.

x11 overlay
The Gentoo X11 team maintains the x11 overlay. It contains X.Org versions in preparation to be moved to the portage tree, pre versions of the next main version and live versions for trunk. Use layman to setup the overlay.