A graphics processing unit (GPU), also occasionally called visual processing unit (VPU), is a specialized electronic circuit designed to rapidly manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display. GPUs are used in embedded systems, mobile phones, personal computers, workstations, and game consoles. Modern GPUs are very efficient at manipulating computer graphics and image processing, and their highly parallel structure makes them more effective than general-purpose CPUs for algorithms where processing of large blocks of data is done in parallel. In a personal computer, a GPU can be present on a video card, or it can be on the motherboard or—in certain CPUs—on the CPU die.
The term GPU was popularized by Nvidia in 1999, who marketed the GeForce 256 as “the world’s first ‘GPU’, or Graphics Processing Unit, a single-chip processor with integrated transform, lighting, triangle setup/clipping, and rendering engines that are capable of processing a minimum of 10 million polygons per second”. Rival ATI Technologies coined the term visual processing unit or VPU with the release of the Radeon 9700 in 2002.
Modern GPUs use most of their transistors to do calculations related to 3D computer graphics. They were initially used to accelerate the memory-intensive work of texture mapping and rendering polygons, later adding units to accelerate geometric calculations such as the rotation and translation of vertices into different coordinate systems. Recent developments in GPUs include support for programmable shaders which can manipulate vertices and textures with many of the same operations supported by CPUs, oversampling and interpolation techniques to reduce aliasing, and very high-precision color spaces. Because most of these computations involve matrix and vector operations, engineers and scientists have increasingly studied the use of GPUs for non-graphical calculations.
In addition to the 3D hardware, today’s GPUs include basic 2D acceleration and framebuffer capabilities (usually with a VGA compatibility mode). Newer cards like AMD/ATI HD5000-HD7000 even lack 2D acceleration; it has to be emulated by 3D hardware.
Video decoding processes that can be accelerated
The video decoding processes that can be accelerated by today’s modern GPU hardware are:
Motion compensation (mocomp)
Inverse discrete cosine transform (iDCT)
Inverse telecine 3:2 and 2:2 pull-down correction
Inverse modified discrete cosine transform (iMDCT)
In-loop deblocking filter
Inverse quantization (IQ)
Variable-length decoding (VLD), more commonly known as slice-level acceleration
Spatial-temporal deinterlacing and automatic interlace/progressive source detection
Bitstream processing (Context-adaptive variable-length coding/Context-adaptive binary arithmetic coding) and perfect pixel positioning.What is the difference between a CPU and a GPU? What is a GPU
An external GPU is used on for example laptops. Laptops might have a lot of RAM and a lot of processing power (CPU), but often lack a powerful graphics card (and instead have an on-board graphics chip). On-board graphics chips are often not powerful enough for playing the latest games, or for other tasks
Therefore it is desirable to be able to attach to some external PCIe bus of a notebook. That may be an x1 2.0 5Gbit/s expresscard or mPCIe (wifi) port or a 10Gbit/s/16Gbit/s Thunderbolt1/Thunderbolt2 port. Those ports being only available on certain candidate notebook systems.
External GPU’s have had little official vendor support. Promising solutions such as Silverstone T004 (aka ASUS XG2) and MSI GUS-II were never released to the general public. MSI’s Gamedock  promising to deliver a full x16 external PCIe bus to a purpose built compact 13″ MSI GS30 notebook. Lenovo and Magma partnering in Sep-2014 to deliver official Thunderbolt eGPU support.