What is Sencha Touch?
What is JQuery Mobile?
"A unified, HTML5-based user interface system for all popular mobile device platforms, built on the rock-solid jQuery and jQuery UI foundation. Its lightweight code is built with progressive enhancement, and has a flexible, easily themeable design." - Jquerymobile.com
If we read the definition carefully, we can see that Sencha is a Framework, that means it provides a complete development stack, including in built composite UI controls, also it has its own UI designer app (commercial) which gives you a kind of WYSIWYG interface for building apps quickly.
While on other hand JQueryMobile is not a framework it's a user interface system in their own words, it does not have its own tools for UI generation (however there are third party web based tools) or their own composite UI controls (may eventually come).