Making an Arduino communicating with another Arduino implies the use of a protocol. Most of the time is a DIY protocol that suits your needs (e.g. sending simple bytes/string etc) made for that project.

[Bill Porter] took care of this problem by creating a library called EasyTransfer

In most of my own projects I define and write my own NMEA standard communication protocols. This makes communications human readable and easy to debug, but proves wasteful with bandwidth and processing power so it’s not right for every application. Binary communications is much more efficient and versatile, but requires careful handling. This library abstracts the finer points of packetized serial communication away from the user so it easy to use and understand.

via [HackADay] source [BillPorter]