Project Fi is Google's own cellular carrier offering, which launched in 2015 and is exclusive to Google's own phones. At first it was only compatible with the then-latest Nexus 6, but has since expanded to be compatible with Google's Nexus 6P and Nexus 5X, as well as the latest Pixel and Pixel XL. Project Fi has also expanded to support data-only tablets, but without the advanced network-switching features available to phones.
Project Fi isn't a traditional carrier in many ways. First of all, it works by combining access to T-Mobile, Sprint and U.S. Cellular's networks, with a SIM card that can actively switch between the three without any user intervention. It also leans on Wi-Fi when possible, and lets you both call and text when connected to a Wi-Fi network. Special software on the phone also automatically connects to open Wi-Fi networks to help limit your mobile data usage, and when it does so also routes your traffic through a Google VPN for safety and connection stability.
The other part of the experience that differentiates Project Fi is its simple and logical billing system. Prices start at $20 per month for the "Fi Basics," which is unlimited talk, text and keeping your account active. After that it's $10 per gigabyte of data you use in the U.S. and abroad in well over 100 countries, no matter what.
Project Fi is a prepaid phone service, so you pay at the start of each billing cycle based on how much data you intend to use, aka $40 for 4GB, and are rebated on the next month if you use less than what you paid for. On the flip side, if you use more than what you paid for you'll simply pay the difference on your next bill at the same standard rate of $10 per gigabyte — no additional overages.
You can also create a "Group Plan" within Project Fi with all of the same benefits mentioned above, but with up to six total people on a single account. The account administrator can manage data access and billing, while individuals can come and go as they please.
Beyond the simple pricing model and unique networking features, Project Fi also offers many of the core features that were available from Google Voice. That means you can forward calls to your Project Fi number on to other phones, as well as make calls and texts from other devices using the Hangouts app.