This is an extremely complex question, as the automobile has been in a state of constant evolution since its proper introduction in 1769 (there were designs for similar vehicles earlier, but no records exist to show they they were ever built).
Simply, the most obvious change in automobiles has been in design and shape; the change from steam and coal to gasoline happened early and has remained mostly intact since (hybrid and electric notwithstanding; neither has proven to be reliable), and other minor changes took years to become standard features. However, the automobile as a status symbol is where most major changes in design occur, even as the technological advances "under the hood" get most of the press. In just the last thirty years, cars have progressed from huge gas guzzlers to tiny compacts, and then to fuel-efficient mid-sizes. Earlier evolutions changed boxy, slow cars to sleek, fast cars, favoring speed over safety.
The large car has often been seen as a symbol of wealth, especially during the 1980s in America, when cars were enormous and very inefficient. As gas prices rose and the world focused on pollution as a real problem, cars shrank in proportion to political pressure; cars in the nineties were often so small they were dangerous to drive. The shift from large to small can be seen both as a cultural norm -- "Everyone drives this size car" -- and a personal reaction -- "I drive this size of car for this reason." In either case, the outward shapes and aesthetics of cars in the 1970s and 1980s became minimized in favor of simple designs intended to show economy and thrift. The famous Toyota Prius, as a hybrid car, has an instantly recognizable shape, and driving one is seen as a testament to how much one cares about his or her environmental impact.
Cars are as much a reflection of the times as they are a reflection of the driver. It is important to examine both the technological advances that make cars faster and safer, and the cultural impact of designs and shifting societal norms.