AddyOsmani has written a really throughout article on Building Single Page Applications(SPAs) With jQuery’s Best Friends; DocumentCloud’s Backbone.js, Underscore.js, LAB.js and jQuery templating.
SPAs are web applications or websites which persistently run in the same page without requiring a reload for further navigation. All of the code needed for your initial load in these applications is driven by either local data or data retrieved from a web server on demand, such as any additional data required from your app that might be driven by user actions.