Bootstrap was developed in 2011 by Mark Otto and Jacob Thornton, a pair of web developers at Twitter.
Here's a quote from Mark Otto's blog about the genesis of Bootstrap:
In short, a super small group of developers and I got together to design and build a new internal tool and saw an opportunity to do something more. Through that process, we saw ourselves build something much more substantial than another internal tool. Months later, we ended up with an early version of Bootstrap as a way to document and share common design patterns and assets within the company.
The most important and fundamental feature of Bootstrap is its grid system. A grid system allows us to properly house our website's content. It divides the screen into multiple rows and up to 12 columns that can be used to create various types of layouts.
Bootstrap also offers many useful HTML components such as navigation bar, panels, badges and dropdown menus. Through its components, Bootstrap helps us add such features to our sites quickly and easily.
Imagine a web where all sites built with Bootstrap look and feel the same. Boring, right? Fortunately, we have the power to change the default styles of any framework we work with.
We can customize Bootstrap by overriding the default CSS. We can customize Bootstrap using Less. We can also customize Bootstrap even before downloading.