Airplay used to be namned Airtunes, but when its functionality grew to include more then music its name were changed.
The Airplay is very much like Bluetooth a way of sending music (data) wireless. The main difference is that Airplay uses/needs a network to send the music and Bluetooth uses radio.
But by using a network, Airplay has a better ranch and you do not have to pair your units. Airplay senders and recievers find each other automatically as long as they are in the same network. This also makes it easy for the user to swith from one Airplay speaker to another.
Airplay 2 is a technology that allows a sender to send to several speakers and thereby create a multiroom system.