I read that MetaFont defines vector fonts. Elsewhere it says it produces raster output. What kind of font does MetaFont really produce?
Fonts with MetaFont are similar to TrueType and Type1 (and OpenType) fonts. Fonts are described in terms of curves and lines but the display is always in pixels, because most used output formats are based on pixels (like screen, laser printer,...). The difference is (among many others, such as hinting) that the place where rasterization happens is different. MetaFont converts these vector fonts into bitmaps right away (the pk-files) and the other font formats are turned into pixels at the system level (like the operating system or the pdf viewer). If the bitmaps are not optimized for the current output device (which is really hard to do nowadays), the output gets ugly. This is why we only use these "vector fonts" today. The rasterizer knows its output device.