A crease is a sharp edge in an otherwise smooth surface. Creasing allows you to combine nice smooth curves with hard edges to make more realistic models. Any edge can be set to be creased. You cannot Crease if the Edit tool is in Selector Mode. The Crease value can be: -1, 0, 1, 2 or 3. A Crease value of 0 means No Crease. A Crease value of -1 means crease with no smoothing. Crease values from 1 to 3 interact with the Smoothness level. The values define the highest Smoothing Level at which the crease is retained. If the Smoothing Level exceeds this value of the crease, the crease is also smoothed. The degree of smoothing is one degree of for each level of difference between the Crease value and the Smoothing Level. For example: If the smooth level of the object is 4, and the Crease value is set to 1, the creased edge will have a effective Smoothness Level of 3.
To crease edges, first select all of the edges you wish to crease.
Once you have your edges selected specify the desired Crease value in the Inspector bar and press Enter.
Since selecting faces also selects their associated edges you can also begin your creasing by selecting faces for the desired edges. This can expedite maters in some cases.