A video card is also known as a graphics card or display card. It is responsible for generating images to a visual display such as a computer monitor or laptop screen. It converts the data into the pixels, addresses, etc. required for display. Sometimes, these cards have chips to perform the conversion.
The video cards can run on the PCI interface, but AGP and PCIe slots provide more resources to the adapter. Now, PCIe is the preferred expansion slot for video cards because the technology of PCIe card is better.
Although the motherboard has some integrated graphics, but a video card is inserted to provide better quality graphics and speed. These cards also provide functions like the rendering of high-quality 3D and 2D graphics. A video card has its own RAM and cooling system which decrease the pressure on the processor and allow more power solely for the display and processing of graphics.