To create a sharp crease, redraw an already existing edge, if you redraw it again it will return to its smooth state. A vertex connected to a sharp edge will be smooth. A vertex with two sharp edges will create a sharp edge and a vertex with three or more sharp edges will create a sharp corner. In general it is wise to keep most edges smooth since it will be most flexible. Even if you have a mesh where all edges are sharp, you may still want to make the edges between two polygons in the same plane smooth, since this makes it easier to change your mind.

Note: that that the middle mouse button tap used to create continuous edges also works to create continuous creases.