Twitter-Bootstrap is the is the World's most well-known responsive front-end and mobile first framework. Bootstrap is the most common structure framework for HTML, CSS, and JS. Front-end framework is all of the stuff that you see when you load an internet page. Responsive is meaning that the webpage itself will fit on any screen (phone screen, computer screen, tablet screen, etc.), and mobile-first is a type of response that trys to fit the webpage that is loading onto a phone screen before trying other screen sizes.
~This webpage is even an example of the bootstrap framework~
How Bootstrap Structures HTML
A web page consists of a header/title menu section, your main body/content section along with any media, and your footer section.
When visualizing Bootstrap, rows and columns are used to structure your content accordingly. You can have up to 12 columns in any single row, there are no restrictions on the number of rows.
Within the divs classified as row, you can use custom classes to define the responsive column section size of the content in your row.
Brief Summary
Twitter-Bootstrap was created by two staff from the developement team at twitter, Mark Otto and Jacob Thorton. It was created to make more of a consistant framework design across internal tools (HTML, CSS, JS). It was released in August of 2011.
Photo Credits: Developement Team.