Bootstrap is an HTML and CSS framework that falls under the ultimate choice of web developers and designers. Of course, for many reasons. It is free, responsive and mobile first. Best of all, it is open-source and is easily adaptable.
Originally, it was developed by Mark Otto and Jacob Thornton at twitter to help the developers and was named Twitter blueprint. Later, when the project started to grow along with a number of developers, then it was changed to Bootstrap. Therefore, we sometimes find bootstrap being referred to as twitter Bootstrap.