Portage

A Portage Introduction

USE flags

Portage Features

Compilation Optimization Guide

Files and Directories

Configuring through Variables

Mixing Software Branches

Additional Portage Tools

Gentoolkit

Portage-utils

You can search by regex using: emerge -s "%^python$"

Uncomment the package you do not want to upgrade in world at /var/lib/portage/world

Overlay
Gentoo Overlays: Users' Guide

Layman homepage

browse overlays that you have not installed

You can search through the ebuilds available in the overlays on http://overlays.gentoo.org/ by using eix.

Defines the directory where overlays should be installed at /var/lib/layman