A Wohngemeinschaft is a flat shared by people who are not a family, each with their own room and a share of the kitchen and bathroom. A WG-Zimmer is one such room. It is not a student-only arrangement or a stopgap: plenty of Berliners in their thirties and forties live in one by choice, and a good WG is one of the better things about the city.
Two structures exist and the difference matters. In a Hauptmieter-WG, one person holds the lease and everyone else sublets from them. In one where everyone is on the lease together, all the flatmates are tenants jointly. As a subletter you are almost always dealing with the first, and often with a room being sublet from someone who is themselves subletting.
The part that surprises newcomers is the selection process. A room in a desirable WG is not let to whoever pays first; it is let after a WG-Casting, an evening where several candidates come round, drink a beer and are quietly assessed on whether the flatmates want to live with them. This is normal and not a formality. Turning up on time, asking the current residents something about themselves and not treating it as a viewing of the furniture gets you a long way.
For short sublets the casting is usually compressed into one message and one visit — but the same logic applies. You are not buying a service, you are being invited into somebody's home, and the messages that get answered read like a person rather than a rental application.