A GPU is specialized for compute-intensive, highly parallel computations.
In order to achieve this goal, compared to a CPU, more transistors are devoted to data processing rather than data caching and flow control.
Single Instruction, Multiple Threads (SIMT)
Nvidia introduced SIMT in the Tesla GPU microarchitecture with the G80 chip.
ATI Technologies, now AMD, released a competing product slightly later on May 14, 2007 (the TeraScale 1-based "R600" GPU chip).
iGPU vs dGPU
iGPU = integrated GPU (integrated into the CPU, that is)
dGPU = dedicated (or discrete?) GPU