Offline data entry/access to CiviCRM
CiviCRM does not have any mechanism which would allow you to make changes locally, offline, and sync later to the master database; what you envision is not currently possible.

A workaround might be to record the updates into a CSV file, then import the CSV file once online.

Theoretically, I suppose one could develop an HTML5 app that stores some subset of database records in browser storage, allows them to be updated, and reads the CiviCRM log or otherwise determines how to reconcile changes made to the live record since the last sync. Even if we hand-wave things like authentication, however, it sounds like a lot of work.