JavaScript (JS) is a versatile programming language primarily used to add interactivity and dynamic behavior to websites. It runs in the browser, allowing developers to create responsive user interfaces, manipulate the DOM (Document Object Model), handle events, and much more.

Key concepts in JavaScript include:

1. Variables: Variables store data values that can be used and manipulated throughout the script. JavaScript uses keywords like var, let, and const to declare variables.

2. Data Types: JavaScript supports various data types, including strings, numbers, booleans, arrays, and objects. Understanding these types is essential for effective data manipulation.

3. Functions: Functions are blocks of code designed to perform a particular task. They can take inputs (parameters) and return outputs, making code reusable and modular. Functions can be declared using the function keyword or as arrow functions.

4. Events: JavaScript can respond to user actions like clicks, key presses, or mouse movements through events. By attaching event listeners to HTML elements, developers can make websites interactive.

5. DOM Manipulation: JavaScript can access and modify HTML elements and their attributes using the DOM. Methods like getElementById, querySelector, innerHTML, and style allow developers to dynamically update the content and appearance of a webpage.

6. Control Structures: JavaScript includes control structures like if-else statements, loops (for, while), and switch cases to control the flow of the program based on conditions.

7. Asynchronous Programming: JavaScript supports asynchronous operations, allowing tasks like fetching data from a server to be performed without blocking the rest of the script. This is often achieved using callbacks, promises, and the async/await syntax.

With these basic concepts, JavaScript enables the creation of dynamic, interactive, and user-friendly web applications.