Designed as basically an ad-hoc service in Apple Inc.’s OS X and iOS operating systems, it was introduced in Mac OS X Lion (OS X 10.7) and iOS 7. Enabling the transfer of documents, videos and images among supported MacIntosh computers and iOS devices such as iPhones, iPads and iPod Touch without using mail or a mass storage device (memory stick / thumb drive). OS X (prior to Yosemite) and iOS (prior to iOS 8) use different protocols and are not inter operable with released versions of Yosemite. The Yosemite released version (version 10.10) of OS X is compatible with previous versions. when used in OS X it operates over wi-fi, whereas the iOS implementation turns both wi-fi and Bluetooth on together. For more information see this link .
There is no restriction on the size of the document that Airdrop will accomodate. Apple users report Airdrop transfers of video documents larger than 10GB. All devices require 10 meter proximity to detect one another.