Javascript is a commonly used implementation of ECMA Script. It's a very similar situation to the Scheme family. In Scheme, you have a highly formalized specification. The current version is R6S6 though R5RS is by far the more commonly implemented standard as of this post. The implementations are more scattered than ECMAScript's due to Scheme's age and general philosophies.
Javascript happens to be common now largely due to the explosive growth in market share of both Mozilla's Firefox and Google's Chrome. It gained a foothold originally with Netscape Navigator which eventually turned into the Mozilla browser and then into Firefox (and Iceweasel if you run with the Scheme crowd).
Now here's where it gets a little dicey. Most of Javascript's performance comes from the parser and engine running it. This is where the real differences between browsers become apparent. Spidermonkey runs Javascript differently than V8 does. Javascript still have rough spots like the lack of a fully common library, though that's being worked on.
So if Javascript were C++, then ECMASCript would be the C++11, with UNIX and G++ being the closest to "standard" while C++ for .NET and Objective C++ lie further afield much in the way Actionscript does from ECMASCript
