Web browsers save data that you viewed while using the web browser to the hard disk (the web browser's cache). Other people can later access this data on your computer's hard disk.
If you use Web SQL, the web browser saves the application data you viewed from the database.
Note that Web SQL does not use cookies for user sessions though.
In order to prevent unauthorized persons from accessing the application data you viewed using Web SQL at a later stage, clear your web browser's cache after exiting Web SQL.
For more information, see the documentation for your web browser.