A motherboard is the base hardware for a computing system.
A central processing unit (CPU) is the main computational processor.
A graphics processing unit (GPU) is a specialized unit capable of parallel processing. It complements (works with) the CPU. There are two types of GPUs — integrated and discrete. An integrated GPU is a graphics processing unit with its own CPU whereas a discrete GPU is a stand-alone circuit board for processing graphics that can be connected to a CPU using a slot. We typically refer to discrete GPUs as “graphics cards.”
GPUs were originally designed to accelerate the rendering of 3D images. A GPU can also be used for other applications like machine learning. NVIDIA GPUs are especially programmable for machine learning applications.
Here is Intel’s page on What is a GPU?