Like it says on the tin.

Zepto.js, a jQuery-compatible mobile JavaScript framework in 2K »

Thomas Fuchs has created exactly what jQuery mobile should be — a lightweight framework that does away with a lot of the cross platform cruft that weighs down jQuery, designed just for mobile. It also does away with all of the fake, timing-based animations that most javascript frameworks use and shifts animations and transitions to CSS3. It’s even got touch events!

I got so frustrated with jQuery mobile a few weeks ago that I started building a mobile mini-framework but have only gotten as far as cloning jQuery’s selector syntax, chaining, event handling and ready listener (if that sounds impressive, trust me, it isn’t). Zepto looks like exactly the sort of thing I wanted, built by people much smarter than I.