Backbone is a framework for front-end JavaScript, unlike jQuery focus on easier DOM manipulation and event binding, backbone provide a structure for separating data model and DOM, just like a MVC framework separate model, view and controller. Make heavy JavaScript application easier to develop and maintain.
Bootstrap, a css framework that developed by Twitter, created by Mark Otto and Jacob Thronton. Which original was the internal framework in twitter for their products and quick prototyping.
On the github page, Bootstrap is described as:
Simple and flexible HTML, CSS, and Javascript for popular user interface components
and interactions.
That is, Bootstrap is not only just a css framework, but actually an UI framework base
on HTML, CSS and Javascripts. It includes the basic css framework that handle details like css reset, typography and grid system. Moreover, it includes the base UI elements like navigation bar, button, form and paginition… and the interaction
UI elements like dropdown, popover and tooltips. Evenmore, all of the color and elements is customizable because the css is generated by less, all setting is in varibles.less file and allow user to change the variables or reuse the elements class in less.