- ECMA-262: The Ecma International (a standards organization) has created the ECMA-262 standard which is the official specification of the ECMAScript language.
- ECMAScript 5: If one talks about ECMAScript 5, one means the 5th edition of ECMA-262, the current edition of this standard.
- Ecma’s Technical Committee 39 (TC39): maintains the ECMAScript language standard (and more). Members of the committee are all major browser vendors and other companies interested in the web. Those companies send delegates to meetings which are also attended by invited experts.
History of ECMAScript versionsThe following are the most currently relevant chapters in ECMAScript’s history:
- ECMAScript 3 (December 1999). This is the version of ECMAScript that most browsers support today. It introduced many features that have become an inherent part of the language:
[...] regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and other enhancements. 
- Develop an incremental update of ECMAScript (which became ECMAScript 5).
- Develop a major new release, which was to be more modest than ECMAScript 4, but much larger in scope than the version after ECMAScript 3. This version has been code-named Harmony, due to the nature of the meeting in which it was conceived.
- Features from ECMAScript 4 that would be dropped: packages, namespaces, early binding.
- Other ideas were to be developed in consensus with all of TC39.
- ECMAScript 5 (December 2009). This version brings several enhancements to the standard library and even updated language semantics via a strict mode. 
- ECMAScript.next (planned for 2013). It quickly became apparent that the plans for Harmony were too ambitious, so its features were split into two groups: Group one are features that are considered for the next version after ECMAScript 5. This version has the code name ECMAScript.next and will probably become ECMAScript 6. Group two are Harmony features that are not considered ready or high-priority enough for ECMAScript.next. Those will still make it into ECMAScript, e.g. as part of ECMAScript.next.next. The current goal is to have ECMAScript.next finished by 2013, with parts of it making it into web browsers (especially Firefox) before then.
Upcoming versions of ECMAScript:
- ECMAScript.next is the code name of the next version of ECMAScript. Using that term implies that one is discussing features that may or may not be added to the final standard.
- ECMAScript 6 is the actual (final) name of ECMAScript.next. Using that term implies that one is talking about features that will definitely be in the standard.
- ECMAScript Harmony is a superset of ECMAScript.next and means “features coming up after ECMAScript 5”. Those features may be in ECMAScript.next, in ECMAScript.next.next or in even later versions.