From Apple:

When you use an iOS app or feature that uses the Internet, your iPhone, iPad, or iPod attempts to connect to the Internet as follows:

• Your device connects to your current Wi-Fi network if available.
• If not available, your device attempts to connect to any nearby Wi-Fi networks you have previously used.
• If there are no previous Wi-Fi networks in range, a list of available Wi-Fi networks appears. You can tap to join a Wi-Fi network, and if necessary enter the password to join it. Note: Wi-Fi networks that require a password are indicated by a padlock icon next to them.
• If no Wi-Fi networks are available or if you choose not to join one, iPhone and iPad (cellular models) connect using an available cellular data network.