Rocket League is more complex than I've described, but not by all that much. An intricate physics engine dictates how your car and the ball move during a match. Where you hit the ball, and the speed at which you hit it, determine where the ball flies and rebounds around the enclosed stadium. Rather than soccer, think of Rocket League as a glorified billiard game, with your car as the cue.
Cars come in several sizes and shapes. The most notable difference is their hitboxes. Larger cars and vans have bigger hitboxes than smaller ones, so hitting the ball is a touch easier with a beefier ride. But there are subtle differences in maneuvering, turn radius, power-slide angle, and so on that make every car unique.