Spotting is a hobby. People who you meet at airports with photocameras,  binoculars, ladders etc. are called spotters.  Their aim is to take photographs or slides of planes. After that they put the slides/photos into their collection or trade it each other. Some spotters travel around the world to take slides of planes at other airports. Planes with special colours are very popular for spotters. They are a rarity and are always a good motif for photos or slides. In Germany there are ca. 1500 spotters.