Diffuse is the main color of the material. This can be modulated with a colored texture.
Transparency can be set by using transparency slider in the diffuse color picker. Textured transparency is also supported by using a transparent PNG image (RGBA) in the diffuse material channel.
To improve the rendering of transparent or semi-transparent surfaces you can do a few things:
Separate the transparent surfaces from the solid surfaces in different materials.
Only use transparent PNG images (RGBA) for transparent materials. Some applications default to saving RGBA PNG images regardless of the alpha channel being used or not. It's better if you save the images as RGB PNG or JPG so p3d.in understands this material is solid.
Double sided transparent surfaces can be problematic. If possible disable this setting from the material properties.
Specularity allows you to control the material's highlight color and intensity. This can be modulated with a colored texture.
Glossiness allows you to control the material shininess. This can be modulated with a grayscale texture.
Normal maps are a way to represent surface detail without the need to increase density in geometry. This textures are generated by specialized 3D software. p3d.in uses the somewhat standard tangent space normal map format.
Bump maps are another way to represent surface detail but this time using simple grayscale images where blacks produce dents and whites produce bumps or vice versa. Bump maps are quite fast to render and reusing the same diffuse or specular texture as a bump will work just fine without adding any extra download time.
Ambient occlusion maps are a trick to recreate the shadowing effect around creases and edges in real lighting conditions. This texture can be colored and is multiplied by the diffuse color.
Glow maps are used to define parts of a model that are emissive and are not affected by regular lighting. This texture can be grayscale or colored and can be influenced by the diffuse color.
Spherical Reflection maps are a trick to simulate reflections from the environment over the model by using a simple "mirror ball" texture. Our reflection maps can be further controlled by Fresnel and separate strenght maps.
Detail maps are special tileable textures used to give the material a second layer of small scale detail. The diffuse, normal and bump channels can benefit from detail maps.
ShadeSpheres are one of those very simple techniques that can produce great visual results. You basically take a small image of a sphere representing the shading of a material. When applied to your model it produces a beautiful and fast representation of almost any material you can imagine.
What if my model has multiple materials and textures?
As soon as your model is displayed in the viewer you will be able to assign different materials and textures to each poly group right in the Mesh and Materials sidebar ( ).
Here's a complete demo on how to set up your model, textures and materials: