What is Javascript ?

JavaScript is the world's most popular programming language. It is the language for HTML and the web, for servers, PCs, laptops, tablets, smart phones, and more.

JavaScript is a scripting language, that is, a lightweight programming language that is interpreted by the browser engine when the web page is loaded. JavaScript is programming code that can be inserted into HTML pages.JavaScript inserted into HTML pages, can be executed by all modern web browsers.

Things you can't do with JavaScript

You can't force JavaScript on a browser.

JavaScript runs in the client, that is, the brower. If you use an older browser without support for JavaScript, or if you simply choose to disable JavaScript in your browser, then a JavaScript script can't work.
Conclusion: unlike what happens with languages that run on the server, such as PHP, you never fully know for sure the impact that the browser your website visitors are going to use will have on your script, or whether your visitors will choose to turn JavaScript support off.

You can't access or affect resources from another internet domain with JavaScript.

This is called the Same Origin Policy. Well, how would you like it if all of a sudden all the nice comments your visitors left on your website started to disappear, or to change place in your page because of a naughty JavaScript script running on another website?
This is exactly the kind of nasty situation that the Same Origin Policy is designed to prevent. Conclusion: your JavaScript script can only access resources in your website.

You can't access server resources with JavaScript.

Because JavaScript is a client-side language, it's limited to what can be done in the client, that is, usually in the browser environment. A JavaScript script cannot access server resources such as databases.

Zillion things you can do with JavaScript

With JavaScript you can:

Put text in an HTML page on-the-fly.
Say you want to display a nice thank you message to a user who has just submitted a comment form on your website. Obviously, the message needs to be added after the user has submitted the form.
You could let the server do that. However, if your website is very busy and your server processes hundreds of forms a day, it might take a little while for your thank you message to appear to the user.
Here's JavaScript to the rescue. Because JavaScript runs in the user's browser, the thank you note can be added and displayed on the page almost instantaneously, making your website users happy.
Make your web pages responsive.
Web environments are dynamic, things happen all the time: the web page loads in the browser, the user clicks a button or moves the mouse over a link, etc. These are called events (which will be the topic of lesson 3).
With JavaScript you can make the page immediately react to these events the way you choose: for example, by showing or hiding specific page elements, by changing the background color, etc.
Detect visitors' browsers.
You can use a JavaScript script to detect the visitor’s browser, or, even better, you can detect what features a certain browser does or does not support. Depending on the browser and its capabilities, you can choose to load a page specifically tailored to that kind of browser (lesson 14).
Create cookies.
A JavaScript script is great if you want to create cookies so that your visitors can enjoy a personalized experience the next time they visit your website (lesson 15).
Validate web form data.
You can use a JavaScript script to validate form data before the form is submitted to a server. This saves the server from extra processing (lesson 16).
And much ... much more.
Learning JavaScript will enable you to add cool animation effects to your web pages without using an external Flash plug-in, use the newest features of HTML5 such as canvas (to draw directly on your web page) and drag and drop capabilities, integrate your website with external web services such as Facebook, Twitter, etc.