Outline rendering
Drawing clean outlines around objects — the hallmark of the toon look and of highlighting a selected thing.
Two families dominate. One is the inverted hull: render a slightly enlarged copy of the model in black with its back faces outward, so only a rim shows around the silhouette. Cheap and simple, per object, but it only catches the outer contour. The other is a post-process that hunts for edges in the depth and normal buffers with a Sobel or Laplacian operator, which also finds creases inside the shape.
It's the backbone of cel-shading, but gets used just as often soberly: the yellow outline around the selected unit in a strategy game is the same trick.