UVs (pronounced U-VEEZ) are two-dimensional texture coordinates that reside with the vertex component information for polygonal and subdivision surface meshes.
UVs exist to define a two-dimensional texture coordinate system, called UV texture space. UV texture space uses the letters U and V to indicate the axes in 2D. UV texture space facilitates the placement of image texture maps on a 3D surface.
UVs are essential in that they provide the connection between the surface mesh and how the image texture gets mapped onto the surface mesh. That is, UVs act as marker points that control which points (pixels) on the texture map correspond to which points (vertices) on the mesh. Textures applied to polygon or subdivision surfaces that do not possess UV texture coordinates will not render.
Although Maya creates UVs by default for many primitive types, you’ll need to rearrange the UVs in most cases, because the default arrangement will usually not match any subsequent edits to the model you may make. In addition, the location of the UV texture coordinates do not automatically update when you edit a surface mesh.
In most cases, you map and arrange UVs after you have completed your modeling, but before you assign textures to the model. Otherwise, changing the model will create a mismatch between the model and the UVs, and affect how any textures appear on the model.
Understanding the concept of UVs and how to map them to a surface, and subsequently lay them out accurately is essential for producing textures on polygonal and subdivision surfaces when working in Maya. This is also important when you need to paint textures, fur, or hair onto a 3D model.
The process of creating explicit UVs for a surface mesh is called UV mapping. UV mapping is a process whereby you create, edit, and otherwise arrange the UVs (that appear as a flattened, two-dimensional representation of the surface mesh, over top of the two-dimensional image to be used as a texture as it appears in the UV Texture Editor.
The UV mapping process results in a correlation between the image and how it appears as a texture when mapped onto the three-dimensional surface mesh. UV mapping is a critical skill to master for accurate and realistic textures on polygonal surfaces.