Common terms and their meanings are listed here to reduce possible ambiguity in this tutorial.

addon

A vpk file containing files that change L4D2. Synonymous with mod. (I use "addon" rather than the noun form of "mod" to avoid confusion.)

HLMV

Short for Half-Life Model Viewer, the model viewer provided in the L4D2 Authoring Tools.

L4D survivors

The survivors introduced in the first Left 4 Dead game (L4D): Bill, Francis, Louis, and Zoey

L4D2 survivors

The survivors introduced in the second Left 4 Dead game (L4D2): Coach, Ellis, Nick, and Rochelle

mesh

The set of vertices and related edges and polygons that determine the shape of the model.

mod

(noun) A vpk file containing files that change L4D2. Synonymous with addon.

mod

(verb) Edit or change L4D2 in some way.

model

Either means the mesh only or the mesh and all the related textures and animations associated with the mesh. This tutorial uses the latter meaning.

rigging

The process of assigning weights to vertices that determines how much each bone's movement affects each vertex.

skinning

This is used as a synonym both for rigging and for texturing, so this tutorial avoids using it.

texturing

The process of assigning image files to polygons in a mesh.

VDC

Valve Developer Community, a wiki that includes information and tutorials about modding Source engine games, edited by both Valve developers and mod community.