Video memory is memory on a video card and in some cases on the motherboard, that's accessible by the video and computer processor. With more video memory, the video card and computer are capable of handling more complex graphics at a faster rate.
The picture above shows that video memory is usually an IC that is part of the video card, not a removable memory module. Video card memory may be as low as 8 MB on older video cards and up to several gigabytes on newer video cards.
Like RAM in a computer, video memory temporarily stores data relating to graphics. As new graphics data comes into the video card, it replaces graphics data no longer needed in the video memory. When a computer is turned off, any graphics data in the video memory is removed.