Impostors is kind of difficult to define, but broadly you can think of it as "abusing the graphics API raster process to draw stuff other than the provided primitives". One of my first OpenGL projects, Vertexture, used OpenGL point primitives (which are rendered as screenspace quads) and discarded fragments at the corners to create circles.
More recently, I've messed with some interesting approaches based on rasterizing a bounding box, and raytracing against the contained primitive in the fragment shader. You can find out more in the articles below: