PNCs known as Philatelic Numismatic Covers or Postal Numismatic Covers are a very flash way of saying "Envelope with a stamp and coin".

PNCs started as an exciting limited release product with one or two being produced per year. Nowadays the market is awash with 30+ PNC releases annually which can make it difficult to keep up with if you are the type of collector who wants to own one of every single release. If you find yourself in that category I suggest finding a theme which appeals to you and start there.