GETTING STARTED WITH XNA: PRIMITIVES

Brecht Kets has published a series of articles about primitives with XNA.

From the articles: "... A triangle is defined by 3 points, which is defined by a Vector3, containing the X, Y and Z coordinate (3D space), and maybe some additional information, such as color. Such a point is called a vertex. XNA has some vertex structures embedded, and one of them is perfect for this example: VertexPositionColor. So let us add this to our class ...".

You can find part I and part II on his blog.