Sense the network: Use the Force.
One of the cool new features in Windows Phone 8.1 is “WiFi Sense” – it’s enabled by default and, in a nutshell, is used to let other people access WiFi networks you already know about, without them having to type in the network password. It also lets you connect to known open networks or secured ones shared by your friends.
If you connect to a network and put in a password, and you’ve allowed WiFi Sense to do so, then your contacts of a given type (who also have WP8.1), will be able to connect to that same WiFi network without needing to know the password. The actual passcode itself is not shared with the contact directly, but it is sent to their phone in a hashed way that means it can be presented to the network for access, without their phone even knowing what the password is. If you’ve only just shared your home network, it could take a couple of days for it to percolate through the WiFi Sense system and show up on your friends’ phones, so take it easy and give it some time.
In practice, this means that if you set a password on your home WiFi, your pals who have WP8.1 will be able to use your home network without 
needing your password (or, in fact, your permission – they’re your friends, after all…). If you live in an apartment block in the city, you might want to be careful about this as you could well have neighbours you know leeching on your broadband, but if you live in a more rural location then perhaps you can trust that the only people within range of your network will be those that you invite onto your property.
The benefit of having WiFi Sense turned on is that your phone will automatically connect to known networks, and use them instead of racking up bandwidth charges on your phone bill (especially handy when travelleing).
The service not only lets you connect to networks enabled by your friends, but open networks are shared by everyone with WiFi Sense switched on (via a crowdsourcing arrangement), and are connected to automatically, accepting T&Cs, providing details like your name & phone number etc. As it happens, the phone comes with a usefully vague set of default information (check it out by going into WiFi settings / WiFi Sense / edit info).
WiFi Sense is available in most countries – for more details or to see more info on how it works, check out the WiFi Sense FAQ.
