Grip between tires and track surface is entirely provided by downforce from the wings, diffuser and all bodywork, really. It's good to have a balance between aerodynamical grip (downforce) and mechanical grip. If you have a lack of mechanical grip your car will be near undrivable in slow corners, and won't perform well at tracks like the Hungaroring . If there isn't enough aerodynamical grip the car will unstable through faster corners, and will suffer in places like Silverstone.
A term used to describe the relationship between the downforce a car generates and the drag it experiences.
Straight line speed depends on aero efficiency. Normally, with more downforce coming from the wings, more drag is generated.
Some specific parts like the diffuser and undertray create downforce with almost no drag penalty (Although downforce with no drag is impossible). This is because the parts are light and barely come into contact with air, compared to the rest of the car. Aerodynamicists tend to refer to aero efficiency in 'points', depending on how much downforce/drag gain there is. 3 'points' is about a tenth per lap