AppleScript is a Mac OS-only inter-application scripting language designed to control and exchange data between scriptable applications.
From the Apple Developer page:
AppleScript is Apple's powerful and versatile native scripting technology for Mac OS X. With AppleScript, you can control, and communicate among, applications, databases, networks, Web services, and even the operating system itself. You can make your Macintosh applications scriptable so that users can write scripts to automate operations they would rather not do manually, from simple tasks to complex workflows involving multiple applications.