Owls were birds which were mainly used in the wizarding world for sending and receiving mail. In the Muggle world, they were just a nocturnal animal. They were intelligent and capable of understanding people. An owl may be owned by a family or employed in the role of a postal owl.
An owl was a very valuable part of everyday life in the wizarding world, so many witches and wizards owned an owl. They could purchase owls in shops including Eeylops Owl Emporium. They were used for communication and delivery. It's unknown how, but an owl always knew where to go to find their owner or the recipient of the post. Owls probably had a code of conduct when delivering, because Hedwig disapproved of Pigwidgeon zooming around when he was happy about a successful delivery.
An owl also delivers newspapers and unless it was pre-paid for, they would have a pouch around their leg for a customer to put money. Hogsmeade had a post office where customers could hire an owl to send a letter. They had at least two-hundred owls, colour-coded to show how fast your letter would get to a destination if you chose that owl.
An owl was long since the way that Hogwarts School of Witchcraft and Wizardry sent an acceptance letter to their students, or a letter every summer that had the names of books and equipment they needed to bring. The letter also tells the student they may bring an owl with them to the school.
In November 1981, the wizarding world celebrated the downfall of Voldemort and sent many owls to each other throughout the day. The increase in the number of owls saw in the Muggle world (and in broad daylight) was noticed and mentioned by the Muggle news. In July 1991, Rubeus Hagrid sent an owl with a note to Albus Dumbledore telling him Harry Potter finally had the acceptance letter that Vernon and Petunia Dursley tried to keep from him.
In June 1994, Sirius Black found an eager owl willing to deliver a letter to Harry Potter. Pigwidgeon was very proud of doing a successful delivery and flew around a train compartment, to Hedwig's dignified disapproval. Ron had to stop Crookshanks eating the owl.