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JScript is an implementation of JavaScript. Other implementations of JavaScript.

From the JScript Homepage (edits mine):

JScript is the Microsoft implementation of the ECMA 262 language specification (ECMAScript Edition 3). With a few minor exceptions (to maintain backwards compatibility), JScript is a full implementation of the ECMA standard.

JScript has some Microsoft-specific extensions meant to aid in using the language as a local programming language on the Windows platform. A short (and probably incomplete) list can be found on the Wikipedia page.