The genre of writing usually involving heavy levels of nanotechnology becoming parts of normal life. Generally these are societies where the singularity should have happened, but for some reason or another the writer has chosen otherwise....
Post-cyberpunk, as the name suggests is a development onwards from cyberpunk works, in that cyberpunk is now "day after tomorrow" stuff, in that we're almost there in terms of cyberwear and information becoming integrated into our daily lives. Post-cyberpunk is looking a little further.
Important authors (IMNSHO):
Neal Stephenson - especially for Snow Crash (a mid-way point between post- and garden-variety cyberpunk) and The Diamond Age
Paul J. Macauley - Fairyland,Secret Harmonies