The app data is stored on your device (your tablet or phone).
It's NOT stored on the internet online. That way the user can use the app without internet connection and avoid the risk that all the customer/business information will be exposured to somebody malicious. Also the app works very quick unlike web based application.
It's not accessble from other devices, however by using "Data Sync" feature, you can synchronize and share the data across multiple devices. (See this FAQ)