Points that lie in the same plane are called coplanar points. Points that do not lie in the same plane are non-coplanar. Two points are always coplanar, but three or more points may or may not be coplanar.
The vertices of a triangle are a good example of three coplanar points. Similarly, a square, rhombus, trapezium and rectangle are two-dimensional plane figures. All four vertices of these figures are coplanar points.
A cylinder is a three-dimensional figure. The points on the curved surface and the points on the bottom of the cylinder do not lie in the same plane and thus are non-coplanar.