Post Punk is a genre that developed after the initial boom of punk. The bands associated with the genre took the DIY ethics of punk and started to experiment with the punk sounds. A perfect example of post-punk is Public Image Ltd. This band featured Johnny Rotten (aka John Lydon)of sex pistols fame. The band was very experimental and added dub influences and krautrock to the punk sound.
Basically Post-Punk is a smart experimental version of punk that originated around 1978.
Examples: Joy Division, Public Image Ltd, Magazine, Gang of Four, The Fall, The Birthday Party