Gouraud is a shading method, named after the French mathematician Henri Gouraud (pronounced on-REE grrr-ROW).
Each triangle's color is created by interpolating the vertex colors that are located at each corner of the triangle. In other words, the interior of each triangle is a smooth gradient between the colors of the three vertices. The vertex colors are usually created dynamically from the lighting in the scene, although the artist can instead assign specific colors to each vertex.
Gouraud shading has a smooth look, but can look strange when using solid colors.