Tres is an implementation of a Backbone.js pattern, which greatly simplifies things by giving views (referred to as "screens") control over all the action that goes on when you're looking at a screen.
This pattern goes really well for mobile web apps, and more so for mobile-specific builds, since it embeds a few best practices suitable for it.
Tres is two-fold: a JavaScript (written in CoffeeScript) part, which you'll need if you plan on writing apps with it, and a stylesheet, which works like a "bootstrap" library for common screen/header/footer/scrollable contents arrangements. You'll need to style your app yourself anyway.
Tres' GitHub repository is an example application in itself, built using Hopla. Hopla is a really simple suite built in a Rake file which gives you Sprockets.
The standalone library (without the repo) can be downloaded from the repo: both tres.js and tres.css.
Tres was initially released as a Ruby gem, and had a set of generators for creating screens, models, and templates. As I increasingly found myself using less of that, and disagreeing with tools like that in principle, I decided to revert the effort into a much simpler library.
That, and the fact I managed to rewrite the entirety of it's back-end features in a single file with a few methods (see Hopla).