jQuery Rewritten, Ground-Up, for Mobile HTML5 on iOS and Android devices. This is how jQ.Mobi defines itself and gives a good insight on what it is.
Compared to jQuery Mobile (24kb), it is much smaller in size with only 3.5kb (both minified and gzipped) and covers most features with the same syntax.
When it comes to speed, jQ.Mobi is impressive there as well and mentioned to be 3x faster on Android and 2.2x faster on iOS. The framework also has a UI library (again smaller in size) and includes plugins for mobile interactions and interfaces