G-force, an abbreviation for "gravitational force", is a force that pulls or pushes on riders. On a roller coaster it is caused when the vehicle has velocity on a curve (either vertical or horizontal). When the train reaches the bottom of a hill, riders typically experience "positive g-forces". When a train crests over the top of a hill, riders experience "negative g-forces". During turns, riders experience "lateral g-forces".
Negative g-force (air time)
Negative g-force is the force that causes the rider to float out of the seat. This is affectionately know as "air time". There are two main types of air time, "floater", and "ejector". Floater air time is is usually g-force a level less than 1 to 0 g, or weightless. Ejector airtime is negative g-force that is measured in a quantity of less than 0 g.
A list of the roller coasters with highest positive g-force can be found here.