Bootstrap is a popular front-end framework for building responsive, mobile-first websites and applications. It was developed by Twitter and is now maintained by a team at GitHub.

Bootstrap provides a set of pre-designed components and styles that can be used to build user interfaces for the web. These components include things like buttons, forms, navbars, and modals, and they are designed to be easy to use and customize.

Bootstrap is based on a grid system that allows developers to create responsive layouts that adapt to the size of the screen. It also includes a number of utility classes that can be used to apply common styles, such as text alignment, padding, and margin, in a consistent and easy-to-use way.

In addition to its core components and styles, Bootstrap also includes a number of optional JavaScript plugins that add interactive features to websites and applications, such as carousels, modals, and dropdowns.

Bootstrap is widely used by web developers due to its ease of use and the wide range of components and styles it provides. It is a popular choice for building responsive websites and applications that work across a variety of devices and screen sizes.