Teachers and students can use this interactive applet to develop a conceptual understanding of the surface area of a cylinder. Users can manipulate a slider to explore the dimensions and area of cylinders. 

Here is net completely closed:

Here is the net completely open:

Users can use the slider to investigate how the cylinder looks in between:

For more ideas on how to develop a conceptual understanding of surface area of cylinders (and other three-dimensional figures), see Three-Dimensional Geometry, Part 1.