Materials define how a mesh is going to be rendered. Materials are more than just a texture that is applied to the mesh. It also defines other things such as how the mesh will interact with the existing scene, whether it will be affected by lighting, and a variety of other settings.
==RenderPass==
Materials are actually a little more complicated than just defining properties that apply to the render system, in fact Materials are little more than a holder for `RenderPass` objects. For each `RenderPass` the `Mesh` is rendered. Why is this useful? By defining multiple render passes you can set up the second pass to interact with the first pass to create effects.
These two images show the output of the MultiPassRenderTest where the alpha of the second pass loops from full alpha to no alpha. The second pass is rendered with additive blending which gives a more intense glow effect.
{F1139}
{F1141}
You can accomplish the same thing by using two meshes and even multiple texture units. This is a trivial example though.
==TextureUnits==
`TextureUnits` are a component of RenderPass.