Deliver xxx cargo to xxx place.
Will instantly fail if you are scanned by NPC authorities.
Will trigger NPC authorities to spawn in outposts to scan you.
Will trigger NPC authorities to spawn in Supercruise mode to interdict you.
Will trigger pirates that attempt to interdict you.
As of 1/3/2016, Shadow Delivery missions are, by a tremendous margin, the fastest way to earn credits in the game, assuming the player has a high enough trade rank.