Bootstrap is open-source framework for create websites and applications. Bootstrap is an open source toolkit for developing with HTML, CSS, and JS with. Bootstrap is the most popular framework for devloping responsive websites.

Bootstrap 4 is the latest version of Bootstrap. More important changes you need to know in Bootstrap 4 comparison of Bootstrap 3.

Less to Sass
Bootstrap 4 has been moved from Less to Sass. Less and Sass only have difference that’s is syntax, both have different syntax with same meaning of code. It also indicates that you will be joining a huge Sass developer community.

Grid system
Bootstrap 4 has 5 grid system (.col-, .col-sm-3, .col-md-3, .col-lg-3, .col-xl-3) and removed the xs grid class from the lowest break point. Therefore, (col-) covers all devices.

.col- (extra small devices – screen width less than 576px)
.col-sm- (small devices – screen width equal to or greater than 576px)
.col-md- (medium devices – screen width equal to or greater than 768px)
.col-lg- (large devices – screen width equal to or greater than 992px)
.col-xl- (xlarge devices – screen width equal to or greater than 1200px)

Flexbox
Flexbox is a new layout mode of CSS3 which is officially called CSS Flexible Box Layout Module. Flexbox quickly manage the layout, alignment, and sizing of grid columns, navigation, components, and more with a full suite of responsive flexbox utilities.

Offsetting Columns
In the Bootstrap 4 move columns to the right using .offset-md-* classes. Bootstrap 4 accepting offset-md-*, but not col-offset-md and the same scenario of offset would be used for other grid system classes.

Responsive Images
Images in the Bootstrap 4 are made responsive with class .img-fluid not .img-responsive.