Bootstrap is a free front-end framework for faster and easier web development It includes HTML and CSS based design templates for typography, forms, buttons, tables, navigation, modals, image carousels and many other, as well as optional JavaScript plugins Bootstrap also gives you the ability to easily create responsive designs
With Bootstrap, web developers can concentrate on the development work, without worrying about design, and get a good looking website up and running quickly. Conversely, it gives web designers a solid foundation for creating interesting Bootstrap themes.
Sass (which stands for 'Syntactically awesome style sheets) is an extension of CSS that enables you to use things like variables, nested rules, inline imports and more. It also helps to keep things organised and allows you to create style sheets faster.The sass syntax uses indentation instead of braces, rules are separated by the newline sign instead of a semicolon used in classic CSS code.
Less (Leaner Style Sheets; sometimes stylized as LESS) is a dynamic preprocessor style sheet language that can be compiled into Cascading Style Sheets (CSS) and run on the client side or server side.[2] Designed by Alexis Sellier, Less is influenced by Sass and has influenced the newer "SCSS" syntax of Sass, which adapted its CSS-like block formatting syntax
Bootstrap has a lot of handy components, from which you can choose and easily integrate or modify to suit your needs as the case may be. Most of these components come with default breakpoints to cater for all screen widths. This can go a long way to help in fast prototyping; a navbar here, a jumbotron there, some forms and containers here and there and you are good to go. The best part is that some components have their javascript behaviour incorporated into them already, so you do not need to worry about trying to write the functionalities for it.
In one sentence; Bootstrap is versatile, simple and very flexible to use. A good knowledge and combination of bootstrap and CSS3 will lead to a very smooth and responsive user interface both to the developers during development stage and the end users when it is viewed live.