Undoubtedly the biggest visual alteration to the cars for the new season is the addition of the 'Halo', the FIA's cockpit protection device - introduced to improve the safety of the drivers in the event of an accident, or when there is flying debris.
The cockpit is a vulnerable place for drivers, as it's the only area where they are exposed. Following several years of development, which included trials during practice sessions, the FIA opted to bring in the Halo for 2018.
The Halo has had its detractors, but it has a clear target and purpose - to drastically improve driver safety. The device itself is mandatory but there is some scope for teams to develop them and gain a performance advantage.