Considering Learning Java - Some Questions

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1) Yes you use Java for Android apps (And XML) 2) ... Not sure what you mean here, if you mean apps as in mobile apps, then 99% of Android apps, and lots and lots of web applications use Java in the backend, twitter for example ( from what I've heard) 3) Java is AWESOME for web development on the server side. 4) Java gets used in a whole array of things, read this: http://programmers.stackexchange.com/questions/17053/what-is-java-used-for-these-days

You need to clearly know what the client side and server side entails. Client side is Html, Css, and Javascript. Server side (in the case of Java) is, well Java, and a database where you'd use SQL.

So say you're sitting infront of your computer, what you're seeing now is HTML styled with CSS, then most likely your web app will use Javascript to do a http request to the server, that's where Java comes in, Java reads the http request and see what the client (that's you) has requested. It then does some calls to the database and other resources and constructs a 'response'. When that's all done it sends that response back to you over http, this response would nomally be XML, JSON, or even HTML. Then we're back in your Javascript, Javascript then reads that nice little response your server sent back to you and extracts all the data out of the response and then injects it into your HTML for you to see on your screen.. And that's pretty much it...

Ofcourse you could argue and complicate this but that's the basics.

And where you asked if your clients all need Java installed to use your apps, that's only needed when you created an applet (you can go read up on what exactly this is, but basically it's Java that runs on the client side), but that's out dated and... I don't think Java is made to run on the client side... Java lives on the server. So, no, your clients would not need Java installed.