What is Renderer? How it is different from a shader?
For Photorealism which one is more important?
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Shaders and renderers are two different things. A renderer is simply a piece of code written on top of a rendering API (such as OpenGL or DirectX) and it handles the model rendering (hence the name).
A shader is also a piece of code, but it runs on the GPU. It's optimized to work on hundreds of threads at once and it's usually used to figure out the position of a vertex (a point of a 3d model) relative to the camera (vertex shader) or to figure out the color of a certain pixel (fragment shader).
Your third question makes no sense currently. None of them are required to achieve photorealism. You can make photorealistic images with just shaders (like here) and you can also achieve it without using a shader, in fact, ray tracing is the current standard in movie making and it doesn't have to involve the usage of shaders.