Materials and textures

The Materials list, in the Textures section, shows the model's materials: each one's color, its texture and how many faces it covers. It lists the materials on the objects shown.

The Textures section: the Preview, and the materials, each with its color, name, faces and texture

Each material

Its color: click the swatch beside its name to pick another. The Base preview shows it, and the MTL file saves it.
Its texture: the box at the right of its row. Drop an image onto it, or click it to pick one. Once a texture's there, click it to take it away again, or drop another onto it. A texture the model names but that isn't loaded yet shows its file name to look for.
Its opacity map, where it has one, named under its texture. See Opacity maps and color keys.

Showing and hiding

Click a material's row to show or hide its faces, in both views; a shown one is lit. Show all and Hide all do every material at once. A face shows only while both its material and its object do.

Textures on the model

Pick File in the Preview list to see each material's texture: on the model, with Textures on in the 3D View's toolbar, and in the UV Layout, under the islands. The UV Layout shows the active material's texture: the one last clicked in the list, or the one the selection is on.

A texture covers the 0-1 square whatever its shape, stretched to fill it, as every renderer lays one on. One with clear parts of its own, such as a PNG's, shows them clear on the model.

Where textures come from

An MTL file, or a .3ds or .i3d model, names each material's texture. Open the images with the model, or later, and each goes to the material that names it. See Opening files.