Blender's Grease Pencil feature allows you to write or draw simple lines in 3D space. While this feature may seem a bit strange at first, it's actually incredibly useful. As an individual, you can use Grease Pencil to quickly sketch out ideas prior to modeling them in Blender. If you're working with a group of people, Grease Pencil allows you to include notes in the 3D View to facilitate collaboration. You can sketch a pose that a character should go into or draw an arc that the surface of a model should follow and pass those notes back to the original artist. You can even do rough 2D animation with this handy little feature!
To use Grease Pencil, simply hold down D while left-clicking and dragging your mouse cursor around the 3D View. The default color for Grease Pencil strokes is black, but you can adjust it, as well as other attributes from Grease Pencil section of the Properties region (N).