A discrete graphics card is an entire secondary or subordinate system -- a small computer, if you like -- that is responsible for delivering the images you see on your computer monitor screen. A typical discrete card consists of a separate, or discrete, graphics processing unit (GPU) and separate graphics memory, although some cards do still borrow from system memory.
GPU
The GPU is a microprocessor specifically designed for processing 3D graphics and is essentially the "brain" of a discrete graphics card. The GPU interprets a series of binary instructions, known as machine code, and converts them into a signal to the monitor. This process involves creating a wire frame model, filling in the pixels and adding lighting, texture and color. It is highly mathematically intensive. The GPU performs the necessary calculations, reducing the burden on the central processing unit (CPU) and freeing processor cycles, which can be used for other purposes.
Performance
Several factors, some of which only become apparent when it is performing certain tasks, affect the performance of a discrete graphics card. The core clock speed of a discrete graphics card -- measured in millions of cycles per second, or megahertz -- determines the speed at which it performs its basic operations, while the memory clock speed determines how quickly information is distributed to the GPU. Some high performance GPUs actually exceed the performance characteristics of high performance CPUs.
Discrete Versus Integrated Graphics
The alternative to a discrete graphics card is integrated graphics, whereby the GPU is attached to, or integrated into, the motherboard of the computer. In this case, the GPU shares resources with the CPU and system memory to display information on the monitor screen. Integrated graphics may be suitable for some 3D graphics applications, but is typically less powerful and slower than a discrete graphics card.
Applications
Integrated graphics may be adequate for Web browsing and basic office applications, such as word processing, but for high performance professional and consumer applications, such as high-definition video or video gaming, a discrete graphics card is often essential. A discrete graphics card is, by its very nature, more power-hungry than a GPU integrated into the motherboard, which can be a concern if battery life is important to you. However, if you're running any form of heavy-duty, graphics-related program, a discrete graphics card typically provides a significant improvement in the quality of the images on the screen, the speed with which they are displayed and overall system performance.
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