Backbone comes packaged in a js file that you can download from backbonejs.org. If you are not planning to hack backbone you should probably download the production version. Backbone can be used with any templating system but it comes with underscore.js support by default. For this reason to use backbone you will also need to download it from underscorejs.org. jQuery is another dependency of backbone, so you also need to include it in your bundle.
In all the examples I show I assume that you have already included jQuery, underscore.js and backbone.js with something similar to: