Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS- and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.

Containers are a fundamental building block of Bootstrap that contain, pad, and align your content within a given device or viewport.

Containers are the most basic layout element in Bootstrap and are required when using our default grid system. Containers are used to contain, pad, and (sometimes) center the content within them. While containers can be nested, most layouts do not require a nested container. The Containers are used to pad the content inside of them, and there are two container classes available.

The Flexible Box Layout Module, makes it easier to design flexible responsive layout structure without using float or positioning.

To start using the Flexbox model, you need to first define a flex container.

The Flexible Box Module, usually referred to as flexbox, was designed as a one-dimensional layout model, and as a method that could offer space distribution between items in an interface and powerful alignment capabilities.

Quickly manage the layout, alignment, and sizing of grid columns, navigation, components, and more with a full suite of responsive flexbox utilities.