Surprising things are completely unexpected—they astonish or startle you. It would be surprising for most of us to get a phone call from the Queen of England.
Unless you suspected your friend was planning something, the surprise party she throws for your birthday will be surprising. Things that are surprising take you aback or even confuse you a little bit. The adjective surprising comes from surprise, via the Old French sorprendre, "to overtake or seize." The original meaning of surprise party was a stealth military action.