On / Off (Wi-Fi)
The most prominent setting in Android's Wi-Fi settings is simply the on / off switch, which enables and disables Wi-Fi on your device completely.
Wi-Fi is the technology behind wireless Internet access over short ranges, such as in homes, workplaces and some public buildings.
While on, your device will search for nearby Wi-Fi access points, and if it sees one that it "remembers" - that is, you have connected to it before and it is still in the list of remembered access points - it will automatically try to connect. If successful, your device will remain connected to that Wi-Fi access point for as long as it is in range.
While connected to a Wi-Fi access point, your device will show a Wi-Fi icon in the notification bar, and all Internet data will be transferred over Wi-Fi instead of over a mobile network. It will therefore not count toward the data use that your mobile carrier sees.
Depending on your settings in the Location settings menu, Wi-Fi may be used by various apps to determine your physical location, even while you are not connected to a Wi-Fi access point. This works because Google maintains their own database of the physical location of various available Wi-Fi networks throughout the world. As of Android 4.3, this can happen even while Wi-Fi is turned "Off" on your device. This should not use significant battery life, but if you are concerned about the privacy implications you can turn this off in your Location settings menu with a setting called Scanning always available.