Airliners and freighters are usually operated by airlines. Each airline usually uses one (or rarely more) airports as hubs. A hub is an airport that the airline uses as a base for all their operations. The airline offices are usually located on the hub airport and the airline operates many flights to and from the hub airport, but very few, if any, flights between other airports.
The major airline using Ruzyně as a hub is ČSA Czech Airlines (the letter Č is pronounced as in Czech). This is the Czech national airline company and a flag carried of the Czech Republic. If you look closely, you can actually see the Czech flag on the ČSA airliners on some of the pictures; the flag is oainted on the tail next to the registration number. ČSA operates flights between Prague and most major European cities and a few locations outside Europe and also cargo and charter flights. You can see the building containing headquarters of Czech Airlines and briefing rooms for their pilots on the following pictures.
The airline code, which is properly called a IATA airline designator, of the Czech Airlines is OK. This code forms the first two letters of flight numbers. (IATA stands for International Air Transport Association.) The callsign of Czech Airlines is CSA-Lines, so the pilots will be using this callsign along with the flight number when talking to air traffic controllers. OK is also a registration prefix of all airplanes registered in the Czech Republic, but this is only a coincidence. Thus, the fact that both the flight numbers of Czech Airlines and registration numbers of Czech Airlines airplanes both start with OK, is also a coincidence - although there's no doubt that one of these letter combinations was created as a copy of the other one. But it's an exception.
Czech Airlines not only operate flights, but they also do other services connected to flying and they also offer these services to other airlines. The most notable kind of service is ground handling of airliners of other airlines on Ruzyně and also aircraft maintenance and repair offered to other airlines.
You can see the Echo Alpha ČSA airliner near a hangar where it's undergoing maintenance. Our tour guide told us it was a Boeing, but it's an Airbus A320-214. As you can see, this aircraft does not have winglets, but wingtip fences, so it must be an Airbus: Airbus airliners of this size (A320 family) have wingtip fences, except the first 21 A320-100 ever produced (ČSA has none of those) which have no wingtip device. Boeing airliners of this size (B737 family) have either winglets (ČSA has none of those), or they have no wingtip device. Unlike winglets, wingtip fences extend off the wing tip both up and down and they are also shorter than winglets, but their goal is the same (although they achieve it in a different way). And if you look carefully, you will notice an orange bar connected to the aircraft's nose wheel; it's the tow-bar used for towing the aircraft and for pushing it back. (And if you still believe the tour guide and not me, you should check the A320 letters on the fuselage of the aircraft; you will rarely find these on Boeing airliners!)
Although the biggest airline at Ruzyně, ČSA Czech Airlines is not the only airline using Ruzyně as a hub. Several more smaller charter airlines operate from Ruzyně, among them a charter airline Travel Service; you can see their headquarters on the next picture.
Copyright notice: Text and images copyright by Michal Řeháček.