Triangles are often particularly nice to work with because they are always planar and simple convex.
A variety of polygon representations can be used, one of the most common ones being an ordered list of references to a vertex list. This avoids redundant storage and redundant computations. We'll also be associating a variety of other information with vertices, such as normals, colours, and texture coordinates. The job of scan conversion is to shade pixels lying within a closed polygon, and to do so efficiently. The fill colour will in general depend on the lighting, texture, and visibility of the polygon being scan-converted.