Rigging

Rigging, or skinning, refers to the act of weighting a polygon mesh to a skeleton for the purposes of animation.

The "mesh" is the skin, and it is stuck to the "bones" of the animation rig, which form a skeleton that can then be manipulated by an Animator to pose and animate the model.

Rigged models include characters, vehicles, weapons, cloth, anything which needs to animate or deform in a game.

The skeleton rig can be a complex structure and is usually recreated to fit the unique requirements of each job. It is usually created by a Technical Artist or an Animator.

Skinning
The term Skinning usually refers to assigning bone weights to the vertices of the model. In the past skinning also meant painting a color texture for a 3D mesh, but this hasn't really survived past the Quake 3 era.

Tutorials

 * Creating Custom Characters for UT3 by Epic Games is an overview of how to setup characters for Unreal using 3ds Max. See also Creating Animations for the Unreal Engine: Character Setup and Rigging.
 * Facial Rigging Tutorial by Alden Filion. Rigging up a  control board with the help of a script.
 * Facial Control Board Tutorial by Jesse Sandifer. A video tutorial that outlines the same method but walks you through it  manually, good info to know.
 * Wrinkle Map Wiring in 3dsMax by Mark "Vig" Dygert
 * 3ds Max - Cloth Sim to Bones by Mark  "Vig" Dygert
 * Multi Layer Cloth Basics by Paul Hormis
 * Creating Collision Meshes Using Conform Wrap by Paul Hormis
 * Using Cloth to Simulate Chains by Paul Hormis
 * Articulation by Brian Tindall, Character Technical Director at Pixar
 * Rigging 101 by Javier Solsona Maya rigging tutorials and resources
 * The Setup Machine Maya rigging made simple
 * MEL Scripting for Animators from Insomniac Games
 * Weight Painting By Kiel Figgins

Free Rigs

 * Animators: Free rigged characters online? from the Polycount forum