AirDrop
AirDrop is a proprietary ad hoc service in Apple Inc.'s iOS and macOS operating systems, introduced in Mac OS X Lion (Mac OS X 10.7) and iOS 7, which can transfer files among supported Macintosh computers and iOS devices by means of close-range wireless communication. This communication takes place over Apple Wireless Direct Link 'Action Frames' and 'Data Frames' using generated link-local IPv6 addresses instead of the Wi-Fi chip's fixed MAC address. 

Prior to OS X Yosemite (OS X 10.10), and under OS X Lion, Mountain Lion, and Mavericks (OS X 10.7–10.9, respectively) the AirDrop protocol in macOS was different from the AirDrop protocol of iOS, and the two were therefore not interoperable. OS X Yosemite and later support the iOS AirDrop protocol, which is used for transfers between a Mac and an iOS device as well as between two 2012 or newer Mac computers, and which uses both Wi-Fi and Bluetooth. Legacy mode for the old AirDrop protocol (which only uses Wi-Fi) between a 2012 or older Mac computer (or a computer running OS X Lion through OS X Mavericks) and another Mac computer was also available until macOS Mojave.

Apple reveals no limit on the size of the file which AirDrop can transfer.

Routine

iOS
On iOS 7 and later, AirDrop can be accessed by either tapping on Settings > General > AirDrop, or via the Control Center. Both Wi-Fi and Bluetooth are automatically switched on when AirDrop is enabled as they are both utilized.
Options for controlling AirDrop discovery by other devices include:
No one can see device (AirDrop disabled)
Only contacts can see device
Everyone can see device.

In iOS 7 or later, if an application implements AirDrop support, it is available through the share button. AirDrop is subject to a number of restrictions on iOS, such as the inability to share music or videos from the native apps.

macOS
On Macs running OS X 10.7 and greater, AirDrop is available in the Finder window sidebar. On Macs running OS X 10.8.1 or later, it can also be accessed through the menu option Go → AirDrop or by pressing ++.

Wi-Fi must be turned on in order for AirDrop to recognize the other device. The other device must also have AirDrop selected in a Finder window sidebar to be able to transfer files. Furthermore, files are not automatically accepted; the receiving user must accept the transfer. This is done to improve security and privacy.

System limitations

Transfer between two iOS devices
Running iOS 7 or later:
 iPhone 5 or newer
 iPad (4th generation) or newer
 iPad Air: all models
 iPad Pro: all models
 iPad Mini: all models
 iPod Touch (5th generation) or newer

AirDrop can be enabled unofficially on iPad (3rd generation). Although not supported by default, AirDrop can be enabled by jailbreaking the device and installing "AirDrop Enabler 7.0+" from Cydia. This procedure is not supported or recommended by Apple, as engaging in jailbreaking can cause software instability, and can introduce viruses.

Transfer between two Mac computers
Running Mac OS X Lion (10.7) or later:
 MacBook Pro: Late 2008 or newer, excluding late 2008 17-inch
 MacBook Air: Late 2010 or newer
 Aluminum MacBook: Late 2008
 MacBook and iMac: Early 2009 or newer
 Mac Mini: Mid 2010 or newer
 Mac Pro: Early 2009 with AirPort Extreme card, or mid 2010 or newer

Transfer between a Mac and an iOS device
To transfer files between a Mac and an iPhone, iPad or iPod touch, the following minimum requirements have to be met: All iOS devices with AirDrop are supported with iOS 8 or later:

Running OS X Yosemite (10.10) or later:
 MacBook Air: Mid 2012 or newer
 MacBook (Retina): all models
 MacBook Pro: Mid 2012 or newer
 iMac: Late 2012 or newer
 iMac Pro: all models
 Mac Mini: Late 2012 or newer
 Mac Pro: Late 2013 or newer

Bluetooth and Wi-Fi have to be turned on for both Mac and iOS devices. (Both devices are not required to be connected to the same Wi-Fi network.)

Security and privacy
AirDrop uses TLS encryption over a direct Apple-created peer-to-peer Wi-Fi connection for transferring files. The Wi-Fi radios of the source and target devices communicate directly without using an Internet connection or Wi-Fi Access Point.

The technical details of AirDrop and the proprietary peer-to-peer Wi-Fi protocol called Apple Wireless Direct Link (AWDL) have been reverse engineered and the resulting open source implementations published as OWL
and OpenDrop.

There have been numerous reported cases where iOS device users with AirDrop privacy set to "Everyone" have received unwanted files from nearby strangers; the phenomenon has been termed "cyber-flashing." Users have the full ability to control their AirDrop settings and limit who can send them files, with options for "Everyone", "Contacts Only", or "Off".

During the initial handshake devices exchange full SHA-256 hashes of users' phone numbers and email addresses, which might be used by attackers to infer the phone numbers and in some cases email addresses themselves.

See also 
 Nearby Share, a similar technology for Android smart phones
 Bonjour, the service discovery protocol employed
 Shoutr, a free P2P multi-user solution for sharing files among multiple people (Wi-Fi)
 Wi-Fi Direct, a similar technology
 Zapya, a free file transfer solution over Wi-Fi

References

External links
 How-to: Use AirDrop to send content from your Mac
 How-to: How to use AirDrop with your iPhone, iPad, or iPod touch

MacOS
IOS
MacOS file sharing software