Owls are known for their ability to fly nearly silently. This is because of the special shape to their wings and the structure of the feathers.
Feathers are often called plumage. Most birds make a sort of flapping, swooshing sound when they fly. But owls have special edge on the front of the wing that breaks air into small streams of wind that rolls to the end of the wing where the comb-like feathers further break the air into even smaller streams. This give them almost silent flight.
The soft fluffy feathers of the legs also help to break up wind noise. The shape of the wing also gives them greater lift which allows them to reduce the amount of flapping required and helps them to sneak up on prey. This video from the BBC helps illustrate the difference in various bird wings and the sound made by their flight.