The "raster" engine is one of the QPaintEngines available inside Qt. When you paint on a QPaintDevice, that device must provide an engine to paint on it.
The raster is a software engine which backs painting on QImages and QWidgets/QPixmaps on Windows (and/or other systems when you set the raster graphicssystem; it'll be the default in 4.8).
X11 and OpenGL are implemented in other QPaintEngines, and used respectively by QWidget/QPixmap under X11 and QGLWidget/QGLPixelBuffer everywhere. (Actually, there are two OpenGL engines, but that's a different story).
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