Viewport Canvas This is a painting system in 3ds Max that makes it possible to paint textures directly onto a model. It was designed to relieve the user from using Photoshop or other paint system to create textures.
Shaders These are sets of instructions that tell the render engine how to display the final render. The instructions come from either the textures used or the presets within or both.
Render to Texture This is a tool in 3ds max that allows the user to render out different layers i.e. normals. Diffuse, specular etc. It also offers the ability to render lighting and shadows into a texture in order to avoid lighting calculations at the point of render. Not to be used if the shadows are animated!