Automobiles are self-propelled vehicles used for travel on land. Most modern automobiles are driven by a gasoline or diesel internal-combustion engine, which is powered by the rotation of the crankshaft. Depending on the design of the car, the engine may be carried in front of the wheels or at the rear. The engine may be connected directly to the wheels, or it may transmit its power to the wheels via a transmission system. The modern automobile usually is designed to carry one or more passengers and a limited amount of cargo. This contrasts with a truck, which is designed to transport goods and is usually much heavier; and a bus (or omnibus) which is designed to transport large numbers of people and often contains cargo as well.

The automobile revolutionized industry, transportation and everyday life in the 20th century. As demand for automobiles rose, ancillary industries such as steel and petroleum developed rapidly. The automobile also spurred the development of better roads and transportation, and services like gas stations and convenience stores sprang up to meet consumer needs. The automobile made it possible for people to move around independently of other people and to visit places that had been inaccessible.

Although many different vehicles had been invented before the automobile, it was the invention of the internal-combustion engine that revolutionized transportation. It allowed for the development of small, lightweight cars with large engines capable of producing enough power to propel them forward. In 1885 Siegfried Marcus, a German engineer, built the first automobile to use an internal-combustion engine. This was a crude model that did not have seats or steering, but it was the first practical automobile. Later, Karl Benz refined the design and produced the Model K in 1900. By 1905, a number of companies were producing automobiles using internal-combustion engines.

Henry Ford, who specialized in mass production, improved manufacturing techniques to lower the price of the automobile. His Model T was the first car to be affordable for middle-class consumers. This and other innovations such as the moving assembly line enabled the automobile to spread throughout the United States.

The automobile stimulated family vacations and led to the growth of tourism-related industries such as hotels, motels and service stations. It ended rural isolation and brought urban amenities, including better schools and medical care, to rural America. The automobile also brought families closer to remote natural landscapes, and it was the catalyst for the development of recreation-related industries such as camping equipment and recreational vehicles.

The automobile has become the primary mode of transportation for most Americans. It is also important in other parts of the world, especially in developing countries. The automobile is responsible for many problems such as air pollution and traffic accidents, but it also has some benefits, such as providing individuals with more freedom and flexibility. Without access to a car, people would have to rely on friends or public transportation for rides, and they would be stuck at home unless they had their own vehicle.