A cylinder has a total of three surfaces: a top, bottom, and middle. The top and bottom, which are circles, are easy to visualize. The third is actually the curved wall of the cylinder. It is a surface that has measurable volume.

A cylinder is one of the most basic curvilinear geometric shapes. Each cylinder has an axis; this is a straight line that passes through the upper and lower surface that makes up the cylinder. The cylinder is bound by two planes perpendicular to the axis. The height of the bounding planes is determined by two fixed points along the axis line, in this case 83 mm.

 
The surface that lies between them is the cylinder. While a circle is 2d a cylinder is 3d having a height(h), radius (r), and volume. Like a circle, a cylinder has a diameter, radius, a chord, a tangent line, and a secant line.

 
When a cylinder is intersected with a plane a 2d circle is the result.
If a cylinder was sliced opened then laid flat it would become:

 A rectangle

A cylinder is a surface. It has volume. If you think of a circle extruded down along a straight line normal to the surface the circle lies on you would have a cylinder.