VEX and VOPs

VEX is the Houdini shading language (it stands for Vector EXpressions) though it can create much more than shaders. And VEX operators (VOPs) is the interactive editor that allows you to create Houdini operators. You can make your own geometry operators (SOPs), composite nodes (COPs), particle operators (POPs), channel operators (CHOPs), and shaders (displacement, light, surface, etc., for Mantra and RenderMan), using the same procedural operator paradigm as you use for modeling, animation, particles, and the rest of Houdini's features.