JSTalk is a scripting language for Mac OS X built on top of JavaScript, with a bridge to Apple's Cocoa libraries.
JSTalk is built on top of Apple's JavaScriptCore, the same JavaScript engine that powers Safari. So when you write in JSTalk, you are really writing JavaScript.
JSTalk also includes a bridge which lets you access Apple's Cocoa frameworks from JavaScript. This means you have a ton wonderful classes and functions you can use in addition to the standard JavaScript library.
JSTalk also adds a preprocessor to make using the Cocoa frameworks friendlier. Since Cocoa is written in Objective-C, you get a different syntax than what you'd normally encounter in JavaScript for calling methods.