Javascript is a widely used programming language that is embedded in most web pages. It convert your static page into one that can interact without needing to wait for a new page to load every time they make a request.

JavaScript is most commonly used as a client side scripting language. The source code is plain text that resides between Begin-Script and End-Script HTML tags in the web page. When a user requests an HTML page with JavaScript in it, the script is sent to the browser and it's up to the browser to do something with it

JavaScript is used in Web site development to do the following:

jQuery is the most popular JavaScript library designed to simplify the manipulation of Document Object Model also known as DOM. The DOM is a tree-structure representation of all the elements of a Web page. jQuery simplifies the syntax for finding, selecting, and changing these DOM element.

It is a lightweight data-interchange format. The format is easy for humans to read and write. And it is also easy for machines to parse and generate. JSON can be validated using the website: http://www.jsonlint.com

An API is a software-to-software interface, not a user interface. Servers can talks to each other without any user knowledge or intervention. When you shop online and enter your credit card information, the web site uses an API to send your credit card information to a remote application that verifies whether your information is correct. Once payment is confirmed, the remote application sends a response back to the web site saying it's OK to deliver the item. The user usually never notice when software functions are handed from one application to another.