JavaScript is a scripting language that allows you to create dynamically updating content, control multimedia, animate images, and pretty much everything else on web pages. It is used to make HTML pages more dynamic and interactive. JavaScript can be used to update and change both HTML and CSS, calculate, manipulate, and validate data, and change HTML content, attribute values, and styles.

JavaScript code can be included in an HTML document using the <script> tag. The <script> element either contains script statements or points to an external script file through the src attribute. JavaScript is often used alongside HTML and CSS to add interactivity to websites.

Some common uses of JavaScript include image manipulation, form validation, and dynamic changes of content. JavaScript is easy to learn and is backed by a strong community of developers.

In summary, JavaScript is a programming language used to create dynamic and interactive content on web applications and browsers. It is used to make HTML pages more dynamic and interactive by updating and changing both HTML and CSS, calculating, manipulating, and validating data, and changing HTML content, attribute values, and styles.