Some people go the airport, not for traveling purposes, but to watch planes. People who visit airports to watch planes for a hobby are called "spotters". These aviation enthusiasts can be seen near runways and at observation decks at airports all around the world.
Interests in aviation are different from spotter to spotter. Some are only interested in civil aviation and won't visit a military airfield, and vise versa. And some spotters are interested in both civil and military aviation.
Most spotters are not just watching planes at an airport. They photograph interesting aircraft and often record the registrations of the aircraft they see. So, cameras and spotters go together well. A collection of several thousand aircraft photos taken by one spotter is not uncommon.
Slides and prints are exchanged during spotters' conventions where spotters meet and talk about their hobby.
Each aircraft has a number painted on it, called the registration of the aircraft. Every country in the world has its own unique code. This unique code, together with a code given by the aviation authorities in a country, makes the registration.
Christian Andersson, Per Christian Reithe, Therese Bejgarn, Peter Tancred and Helgar Nilsson at Stockholm Arlanda runway 26.

The registration of a Swedish plane always starts with SE-. This unique number is interesting to spotters. Most spotters collect registrations, perhaps with the goal of logging them all. Some spotters want to see all planes of a particular type.