This is pretty much from the maya docs and explains it very well....

A polygon face is planar when:
all of its vertices lie in a certain plane. For example, a triangle face is always planar, because its three points define a plane.

A polygon face is non-planar when:
it has more than three vertices, and one or more of those vertices do not lie in the same plane. When a polygon mesh is comprised of quads or n-gons it is possible to have non-planar polygon faces.

In general, you should try to create planar polygon faces on your polygon meshes whenever possible. Non-planar polygon faces may render incorrectly in your final images or when exported to an interactive video game console. For example, a visual shaded line or an unwanted spike may appear on the polygon mesh where it has been subdivided for rendering because it is non-planar.

Maya provides the following features to help you avoid non-planar polygon faces in your polygon meshes:

1. 1 To avoid creating non-planar polygons you can use the Keep New Faces Planar option when using the Mesh > Create Polygon Tool and Edit Mesh > Append to Polygon Tool features.
   2 To correct non-planar polygon faces you can use the Edit Mesh > Split Polygon Tool.
   3 To visually identify non-planar polygon faces you can customize the display of polygons by selecting Display > Polygons > Non-planar Faces.