File types
There are different image formats you can render to, you can group them into lossy and lossless categories:
Lossy: JPG, GIF
Lossless: PNG, TIFF, RAW, BMP
Rendering 3d content to EXR is the most common. Which is a lossless format and can contain multiple render passes in one file.
AOVs
When rendering, the image you get by default is a beauty render. In most rendering enginesm you can render AOVs alongside the render
Arbitary output variables and is a render pass that can be any of the following
Direct and indirect light passes
diffuse, specular, sheen, subsurfaces
position, normal, depth
We can include these by rendering to an EXR file
Multipass Compositing
Compositors are often required to make image adjustments after the CG content has rendered. They are able to do this by building the render from the AOVs alone and blending modes to merge them. This allows artists to control elements like lighting and specularity without having to re render.