Barajas Airport, Madrid. Source: Jean-Pierre Dalbera under Creative Commons license.

The speed at which the Internet evolves can make milestones from 10 or 15 years ago seem worthy of encasement in a glass museum display. Take this quote from Priceline Group's first annual report after its 1999 IPO:

Priceline.com commenced its service with the sale of leisure airline tickets. The number of airlines participating in priceline.com's airline ticket service has increased to a total of 10 domestic airlines and 20 international airlines.

The online travel industry has grown exponentially since those heady days when Priceline managed to sign up 20 international carriers, not to mention pitchman William Shatner. While traditional travel agencies, especially those that cater to business travelers, haven't disappeared, most of us now find it second nature to book travel on our desktop computers, laptops, and mobile devices.

What is the online travel industry?

Companies that facilitate purchases of flights, hotel rooms, rental cars, and travel-related activity over the web comprise the online travel industry. The industry includes well known travel services such as Expedia, TravelocityTripAdvisor, and Orbitz Worldwide. It also encompasses newer, smaller companies, often competing on the basis of incremental innovations. For example, travel site Hipmunk.com presents airline query results in a visual format, ranking results not only by price but also by travel length. 

How big is the online travel industry?

The online travel industry is a subset of the global travel and tourism industry, which, according to Statista.com, had a direct impact of $2.2 trillion on global GDP in 2013. U.S. revenues of online travel companies were estimated at $157 billion in 2013. Revenues of the global online travel industry, sometimes referred to as the global "digital travel industry," are estimated to be between $400 billion-$500 billion annually. With increasing Internet usage worldwide, we can expect this market to continue to expand, especially in developing markets such as Latin America.

How does the online travel industry work?

Corfu, Greece. Source: iwiseguy71 under Creative Commons license.

The online travel industry is divided into three primary categories: suppliers, online travel agencies (or OTAs), and aggregators. Suppliers are the airlines, hotels, and rental car companies offering their services to businesses and individuals. Suppliers sell services directly to consumers via their own websites, but also widely utilize OTAs and aggregators to market their inventories. OTAs provide suppliers' pricing to consumers and fulfill online orders. Aggregators provide a means for web users to compare prices of OTAs and suppliers for specific travel queries, routing users to back to these organizations for purchases.

In recent years, major OTAs like Priceline and TripAdvisor have extended their revenue base by purchasing aggregators, blurring the line between the two business models. Priceline owns aggregators Kayak.com and Booking.com. TripAdvisor counts Airfarewatchdog.com and BookingBuddy.com among its properties. 

OTAs and aggregators rely on both organic and paid search (i.e., searches for travel sites on search engines like Google) as well as other advertising spends to capture customers. Priceline and Expedia are by far the largest digital advertisers; according to e-commerce research organization eMarketer, Priceline's 2013 global spend of $1.8 billion was equal to over half of all digital travel advertising spend in the U.S.

What are the drivers of the online travel industry?

Several trends and financial factors drive the online travel industry. Most prominent is global economic growth. As you might expect, rising discretionary incomes play an important role in the industry. However, as OTAs receive commissions on sales, the direction of hotel room rates and airline rates also affects revenues. Rising average daily hotel room rates since the recession of 2009, for example, have benefited OTAs' top-line revenue. 

Meta-search, the process by which an online travel site includes results of several different OTAs on a single page for easy comparison, also drives this industry. The convenience of meta-search results has propelled the rise of aggregators and is partially responsible for the recent popularity of aggregators as acquisition targets by traditional OTAs.

Perhaps the most noticeable trend driving the online travel industry is the shift from desktop computing to mobile phones and tablets. The general tilt in the population toward "mobile" usage is having a marked impact on the online travel industry. According to industry research group PhoCusWright, mobile phones and tablets made up 20% of online travel spending in 2013. As this share of the total industry rises, OTAs and aggregators will invest significant resources to optimize their interfaces for mobile devices.

The growing tendency for digital apps to foster consumer-to consumer transactions will also influence the online travel industry in the near future. Home sharing site airbnb.com raised nearly $500 million of private venture capital investment in 2014, at an impressive valuation of more than $10 billion. The interest of Silicon Valley in pioneers of the sharing economy like airbnb indicates that new breeds of travel sites -- bypassing both suppliers and OTAs -- have the potential to unsettle the business model of this still-young industry.

Finally, long-term capacity trends in the airline industry will drive online travel opportunities for years to come. Expansion in the online industry has occurred in tandem with the falling cost per mile of air travel to consumers, as airlines have revamped their fleets with lighter, more fuel-efficient aircraft and focused on lowering fixed costs and increasing profitability. Air travel is vital to the online travel industry, as healthy aviation traffic drives not only sales of flights, but hotel stays and rental car bookings as well.