An airline is a company which offers air transport for passengers and freight. Airlines vary widely in size, from small companies with only one or two planes to huge multinational companies. While the industry initially grew slowly, airlines began to take off in a major way after the Second World War, thanks in part to large numbers of returning aviators. By the 1990s, air travel had become ubiquitous.

Different airlines operate in different ways. Some, for example, purchase their aircraft, customizing them to offer special services such as reclining beds for passengers. These companies staff their planes with pilots and cabin crew who train to work specifically for that airline. Others lease their aircraft and staff from other companies. In either case, an airline is heavily supported by a large ground staff which includes reservation agents, cleaners, caterers, and loaders, among many others.

When an airline carries passengers, it establishes a series of regular routes along with a consistent schedule. Most of these routes are set up on the hub and spoke model, with the airline having a central hub through which many flights are routed. Passengers can purchase tickets directly through the airline, or through licensed resale agents. Most passenger air carriers also sign codeshare agreements, allowing them to cooperate with other airlines to offer more flights.

Freight airlines also typically follow a regular schedule through a myriad of set routes which are designed to efficiently move packages from one place to another. A freight carrier may supplement its aircraft with delivery trucks, or it may only offer bulk transit from one airport to another, with customers responsible for moving the freight to and from the airport. The cabins of these aircraft are also laid out very differently than passenger aircraft, allowing staff to pack the planes with freight and packages.

Sending people and goods by air is a rapid and efficient way to get them from place to place. When the airline industry became more widespread, prices also dropped, making it a reasonably affordable mode of travel, as well. Government regulation has also increased, to ensure that air travel is safe and secure for people and goods. International standards are also enforced, since many airlines offer flights between numerous countries, which may have varying degrees of internal regulation. Different airlines have different specializations, including cheap domestic flights, luxury intercontinental travel, or overnight global shipping for important packages.