As explained by JavaScript guru Douglas Crockford in his talk titled The JavaScript Programming Language on YUI Theater,
MS did not want to deal with Sun about the trademark issue, and so they called their implementation JScript. A lot of people think that JScript and JavaScript are different but similar languages. That's not the case. They are just different names for the same language, and the reason the names are different was to get around trademark issues.
JScript supports conditional compilation, which allows a programmer to selectively execute code within block comments. This is an extension to the ECMAScript standard that is not supported in other JavaScript implementations.
Source: Wiki
