I had this at the old 3-DA texturing forum:

What is UV-Mapping ?

In plain English, it's preparing a model-mesh, so you can apply textures to it. For those interested in the science bit, it can be defined as flattening and mapping your 3D mesh, in order to create a template that can be used to apply textures. Since the XYZ parameters in generic 3D environments already are taken, UV-mapping takes the set of letters precluding XYZ, where;

U is horizontal and corresponds to the X-axis (right, left) in your 3D-app

V is vertical and corresponds to the Y-axis (Up, Down) in your 3D-app

W is depth, and corresponds to the Z-axis (Front, Back) in your 3D-app

UV-space goes from left to right, starting in the upper left corner at 0,0 and ending in the lower right corner at 1,1

You can map in the following directions, starting with the most common one:

Planar
Use this when you only want to map one side of an object, or if you've divided your UV-map into a typical Poser front and back-part; Planar mapping projects the map from a single plane flat against the object, somewhat like projecting a slide. If you're a stickler for detail, also use it for mapping multiple sides, and for mapping two sides of a symmetrical object. If you've ever taken a look at bad poser textures from the side, you now know why there are seam-guides. Planar isn't optimal for making things line up perfectly w/o tweaking.

Cylindrical
Typically used on sleeves and (in my case) legs on pants; projects the map in a cylindrical shape, wrapping it around the mesh.
Cylindrical isn't optimal to use when it comes to mapping tops, etc, because the edges are so much more visible. You can of course cure this by using a seamless texture and a fill parameter, but sometimes that's just not what you want.

Some programs have a separate option for the Cap parameter, which is a pretty handy one it applies planar mapping to the caps of the cylinder.
Careful, though: If the ends the mesh you're working with are not at right angles to the sides, using Cap may make it bleed onto the sides of your mesh.

Spherical
Planet mode ! Wraps texture as if the mesh you're using is a sphere. Seams are visible at the poles, unless you do the seamless thing. Good for plastic beach balls.

Box
Six-sided map, each sub-map in planar mode. Faces are mapped from the closest box surface whose normal most closely parallels its own normal.
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And then I went on to sing LithUnwrap's praises.
Which I still do , btw.

~S
(edit - removed site-specific reference)

Message edited on: 07/09/2004 21:21