Updated for Blender Version: 3.1
To the Point
- Select Object (an object with a SubSurf modifier) and enter Edit mode
- Turn on Edge Select mode
- Shift+Click to select edges to crease
- Either press Ctrl+E > Edge Crease, or press Shift > E
- Drag left or right until satisfied with crease, then left-click to complete chaannges
The Details
The crease tool comes in handy when working on objects that need both hard edges as well as Subdivision Surface modifiers. Take a simple cube for instance. When a subsurf modifier is added to it, it more closely resembles a ball.
A crease is needed along the edge or edges to sharpen up some of this object. One way to do this across your whole object is to press N to bring up the Properties shelf in the 3D viewport, then find Edges Data > Crease (or Mean Crease if ALL edges are selected). This is the average crease applied to all selected edges of your object (0 being no crease, and 1 being total crease). This can either be applied to all edges (by selecting the whole object) or by selecting specific edges to be creased.
There is another way to apply creases to your object. Using Edge Select mode in Edit Mode, select the edges to be creased using Shift+Click. Press either Ctrl+E > Edge Crease or simply Shift+E, then slide your mouse left and right to increase or decrease the amount of crease. This is the same effect the slider in the Properties shelf under Edge Data. Left-click to apply the crease amount.