The US Open -- as well the US Women's Open, US Senior Open and the US Senior Women's Open -- has a new playoff format starting in 2018. The USGA has done away with the old 18-hole Monday playoff where the total score determined the champion, and they've moved to a shorter, aggregate-score playoff like we've seen in the British Open since 1989 and in the PGA Championship since 2000.
The new US Open playoff format is a two-hole, aggregate-score playoff which begins immediately after the completion of 72-hole stroke play. Whichever players are tied for the lead at the end of regulation will play two holes at the host course, and the total score will determine the winner. This isn't different than the British Open playoff format or the PGA Championship playoff format except that the US Open is two holes, the British Open is four holes and the PGA Championship is three holes. (The Masters playoff format is sudden death.)
In the event of a tie after the two holes, the tied players will then enter sudden death to determine a winner.
The USGA made this change for all their open championships, ending a three-hole playoff for the US Women's Open and a four-hole playoff for the US Senior Open. They did this after talking to stakeholders, including players, TV rights holders and fans, all who want to see a US Open winner on Sunday rather than the past of waiting until noon on Monday for an 18-hole playoff to start.
Going to a two-hole playoff has caught some players by surprise, including 2015 US Open winner Jordan Spieth. He was asked about the two-hole format at the 2018 US Open at Shinnecock Hills, and Spieth indicated he had no idea the format had changed.