Java is a "language" developed by Sun Microsystems to be "universal" in all operating systems and on all computers, with an appropriate simple "interpreter." Structurally, it may be thought of as "like html" although as a programming language it is much more powerful and can do about anything to anybody when used by someone malicious.
Microsoft had an "agreement" with Sun (according to Microsoft) that allowed them to include Java as a "free option" in Windows. Sun disagreed about the option, so there were continual exchanges of "Did too" - "Did not" - "Yes ya did" - "No we didn't" for a few decades, but nothing very serious ever came of it.
Java has always been considered too powerful to be really safe.
When Sun decided to "restructure" they sold Java to Oracle. Oracle immediately demanded "per copy royalties" from Microsoft, so it ceased being a free option in Windows. Oracle failed to provide useful support, so Java almost immediately displaced Flash (the previous leader) as "the source of all malware attacks."
Some credible estimates are that in 2012 more than 50% of all successful malware exploits used Java.. A recent estimate was that it would take Oracle, if they worked hard on it, at least two years to patch known vulnerabilities - if no new ones were found.
New ones have been found.
There is a growing concensus that "home users" almost NEVER NEED JAVA AT ALL. I've seen only ONE program identified that might be fairly widely used "by the public" that requires JAVA. It's a particular one for stealing downloading YouTube videos, and there are non-Java alternatives.
Some businesses may have programs that require it, but they should know, or have advisors to tell you, if you must have it and when you need to turn it on. Otherwise, many advisors recommend that you just get rid of it.
JScript is not the same thing as Java. It's just an "interpreter" that can read a number of different kinds of "scripts" in much the same way that your browser "interprets" html.
JSript originated as "LiveScript," produced by a company whose name nobody remembers; but another company argued about the naming so both companies changed their name to spite each other and both disappeared soon after. Since the interpreter was left in limbo, it was tacked onto Java (as an app) and renamed JScript. Downloading Java has been an easy way to get JScript, and the JScript has been about the only part of Java most people ever used, but there are other "script readers" that apparently are able to handle JScripts in web pages quite adequately. Information on which one does it in which case is vague, and there are multiple suspects.
When one of the declarations of disaster appeared several months ago, I made sure that I removed all prior Java installations and got and installed - from the source - the latest and best of the whole thing. Installation was verified.
When I recently tried to check what versions I have, I found the answer to be - - - - NONE.
ALL JAVA APPLICATIONS AND APPLETS HAVE DISAPPEARED FROM MY COMPUTER - and I didn't even notice that they were gone. I don't know whether Windows Malware Remover zapped them, whether Norton 360 removed them as "malware," or whether a Windows or IE update just took them away. JScripts still do what they say they do, and everything else still works as well as before.
(If they'd do the same to Flash I'd have another celebratory brew.)
[For info: I'm running Windows 7 Home Premium and Norton 360, with "AutoUpdates - install automatically" for both.]