On the most basic level, an airline alliance is an agreement between many airlines to work together to share flying routes and resources, and extend reciprocal benefits to one another’s frequent fliers.
There are three big airline alliances: Star Alliance, oneworld, and SkyTeam.
The following airlines are part of the Star Alliance:
The following airlines are part of oneworld:
The following airlines are part of SkyTeam:
The Star Alliance is the biggest airline alliance in terms of number of participating airlines, with 26 airlines in total. However, the SkyTeam alliance actually transports more passengers each year (730 million to Star Alliance’s 728 million).
Members of airline alliances work together to share everything from flight routes to airport lounges, ground crews, and even their booking systems. An alliance benefits the airlines financially, but it also helps travelers by providing routes to more places in the world without you having to book separate flights.
Airline alliances have several benefits for both the airlines and passengers. By working with other airlines, an airline can expand its route network to be much larger than what would exist with any one airline.
When things go wrong, an airline that’s part of an alliance has more options; for example if you miss your connecting flight on United, you could be put on the next flight operated by any of their alliance partners that gets you where you need to go.
Airline alliances also allow bag interlining (you can check your bag with Delta and it’ll be checked through to your connecting flight with Air France, for example), and they allow you to use your points accumulated on one airline to travel on an allied airline. Plus, you'll still earn miles with your airline of choice. Finally, you'll have access to other airlines' lounges. For example, oneworld elite members have access to over 650 airport lounges around the world.
A codeshare occurs when two airlines realize that there's a specific benefit to working together and sharing their "codes" on each other's flights. Similar to an alliance, codesharing helps generate more business for the airline, especially when an alliance doesn't have flights flying to a specific destination, and gives travelers more options.
For example, although Cathay Pacific flies to Toronto and Vancouver, it doesn’t offer flights to smaller destinations in Canada, and neither do any of its oneworld alliance partners. For example, if you wanted to fly from Hong Kong to Calgary, you could fly Cathay Pacific to Vancouver, but then you’d have to book a separate flight from Vancouver to Calgary. By codesharing with Air Canada even though they’re in different alliances, Cathay Pacific can give its passengers access to more destinations.
Sometimes on your ticket, you'll notice an "operated by" beneath your itinerary. This text is how you'll know you're on a codeshare flight (although you'll also be informed of this when you first purchase your ticket). For example, you may have booked a flight to Hawaii on American Airlines, but you'll see "Operated by Hawaiian Airlines" somewhere on your ticket.
If you're wondering about the baggage policy across different airlines: the policy of the carrier operating the flight (or the first carrier, if you have connections on multiple airlines) is usually the policy that applies.