Wireless vs data connection
Recent Android versions have a setting to let you designate some Wi-Fi connections as "metered"; i.e. they're actually a hotspot for which you get charged per byte transferred.

On vanilla Android, go to the Settings app, then Data usage, then on the overflow menu, choose Network restrictions. The name isn't very obvious, but the next screen explains:


  Metered networks are treated like mobile when background data is restricted. Apps may warn before using these networks for large downloads.


Within apps, any "only on Wi-Fi" options are affected by these settings: the networks you select do not count as Wi-Fi for that purpose.