Buy on board

In commercial aviation, buy on board (BoB) is a system where in-flight food or beverages are not included in the ticket price, but are either purchased on board, or ordered in advance as an optional extra during or after booking process.

How to buy
It is available for purchase while navigating from takeoff to landing posture.
A product catalog (monthly update) is placed near the seat, and passengers make purchase requests to the cabin crew in the seat on that basis.
Pay by cash, credit card, coupon issued by airline etc etc.
Goods wrapped by the cabin crew are handed over.
In addition to selling in the airplane, some airlines can also purchase through airline website, mail order by sending purchase-only postcards.
Examples of goods to be sold
In major airlines, it is often the original product of an airline company such as brand watches, bags, wallets, accessories, perfumes, pens, cosmetics, and sometimes handles limited items only on in-flight sales. On international flights, duty-free goods such as liquor and cigarettes are also being sold.

Cheap airlines sell alcoholic beverages, soft drinks and light snacks as well (in-flight meals are often charged separately to make the fare even cheaper).

United States
Starting in 2003, many United States air carriers began eliminating free meal services in economy classes on North American flights and replacing them with buy on board services. By 2009, many US carriers had established buy on board as part of an à la carte pricing movement. Around that year, US carriers began using celebrity-named and brand name products to make their buy on board products generate more revenue. Continental Airlines, the last large United States carrier to offer free meals on all domestic flights, announced in March 2010 that it would begin a buy on board program in fall 2010 and end many of its free meal programs on domestic flights. Jeff Green of Businessweek described the end of Continental’s program as an “end of an era.”

As the airline market in the United States became deregulated, airlines began to compete by price. Airline ticket prices began to decrease, and airlines began to charge extra for services that were once included in the airfare.

In the United States, passengers increasingly began to bring their own foods on board to avoid paying for buy on board. In the 2000s US Airways (now part of American Airlines) briefly charged for soft drinks, but then reversed course.

As of 2016, Hawaiian Airlines remains the last U.S. legacy airline to offer free meals on board, although all of its flights are to/from Hawaii. Southwest is truly the only U.S. airline without a buy-on-board program as of 2016. Today, all 3 major U.S. airlines now offer free snacks in economy on board their flights, in addition to their buy-on-board menus.

Europe
In Europe, the general increase in number of tourists that fly, and deregulation which enabled low price carriers, has caused stiffer price competition. Especially the low price carrier Ryanair, who does not give away any food or drink but sells it, has paved the way for traditional airlines to lower their cost. Most traditional airlines still (2016) give a piece of bread or snack and coffee or tea to economy class passengers and sell better food.

Source from Wikipedia