The Draw Nodes
Nuke is not primarily an authoring application. That is to say, it assembles stuff authored elsewhere as opposed to making it. Nonetheless, there are a few nodes whose job it is to make things and most of them live in the draw tools menu. Most of the time these nodes will be used to create simple grey scale information that will be used to mask color operations and suchlike.
A page on the ins and outs of rotoscoping.