CPU, or μP, is the Central Processing Unit (MicroProcessor)...the "brain" of the computer and, often, the largest single integrated circuit on the motherboard. If you don't have this, you don't have what we call a computer.
GPU is the Graphics Processing Unit, or video card, as you will hear it referred to as.
dGPU is a Dedicated Graphics Processing Unit, which is a separate card that is inserted into one of the Input / Output (I/O) expansion slots on the motherboard.
iGPU is the Integrated Graphics Processing Unit, which can be contained either in a discrete graphics processor chip soldered to the motherboard, integrated into the motherboard chipset, or included within the CPU; in which case the CPU is then called an APU instead of a CPU.
You need both a CPU and some sort of implementation of graphics processor (iGPU or dGPU....or the APU) to make what we consider to be a useful desktop or portable computer.