A tortillon, or blending stub, is tight roll of paper used to smear graphite (or any other medium). The idea is that a smoother gradation can be achieved this way. Depending on the style of art, a tortillon can be a great asset to an artist's arsenal. More often than not, however, the tortillon is over used and leaves the picture more blurred, like a crappy JPEG. A brief glance around a high school drawing class will illustrate this point quite well.
This noble and often misunderstood tool is very useful when making shiny surfaces and touching up fine details but should probably be avoided when other textures are more appropriate. For example, when drawing hair, the tortillon is probably not the right tool for the job.