A vertex shader is a type of computer program used in graphics processing to handle the properties of 3D models. It processes each vertex's data, such as its position, color, and texture coordinates, transforming them into a format suitable for rendering on the screen. This transformation is essential for creating realistic images in video games and simulations. Vertex shaders are part of the graphics pipeline, which is a series of steps that graphics data goes through to be displayed. They work in conjunction with other shaders, like fragment shaders, to produce the final visual output.