JavaScript is an interpreted prototype-based object-oriented scripting language without any classes like other object-oriented languages such as Java or PHP (Right!  There are no classes but only prototypes in JavaScript) . It was developed by Netscape

It is dynamic, weakly typed.  It has C-like syntax with Java-like  names and naming conventions, but it is actually very different from or unrelated with Java whose name was borrowed for marketing purpose.

Although Javascript can be used outside of web pages (in PDF etc.),  it is the most famous for its wide usage to create dynamic interactive Web application within most modern Web browsers such as Chrome, Firefox and Internet Explorer.

We focus on client-side JavaScript, which is used as part of a web browser.  The default script for all Web browsers is JavaScript or JS, that is why we can simplify the following

Or into Or  by omitting the type attribute. 

More official readings on JavaScript

JS Versions

JavaScript 1.0:  March 1996, used in Netscape Navigator 2.0  and IE 3.0

JavaScript 1.5:  November 2000   equivalent to ECMA-263 3rd Edition,  used in Firefox 1,  IE 5.5 to 8 and Chrome 1 to 10.

JavaScript 1.8.5 (most current one maybe 1.9): July 27, 2010, equivalent to ECMAscript 5 compliance.   available in Mozilla Firefox 4  and IE 9

Note: to check which version the browser supports, use this code:

<script type="text/javascript">  var jsver = 1.0;</script>
<script language="Javascript1.1">  jsver = 1.1;</script>
<script language="Javascript1.2">  jsver = 1.2;</script>
<script language="Javascript1.3">  jsver = 1.3;</script>
<script language="Javascript1.4">  jsver = 1.4;</script>
<script language="Javascript1.5">  jsver = 1.5;</script>
<script language="Javascript1.6">  jsver = 1.6;</script>
<script language="Javascript1.7">  jsver = 1.7;</script>
<script language="Javascript1.8">  jsver = 1.8;</script>
<script language="Javascript1.9">  jsver = 1.9;</script>
<script type="text/javascript">  document.write(jsver);</script>

is the JavaScript Version of your browser.