Tribes - High level products (hotels, flights, car hire)
Chapters - "Departments"; areas of expertise (data acquisition, front end)
Squads - Autonomous project units (New features, development of an existing feature)
Guilds - Informal interest groups (Linux, Python, agile development)
Everyone is in a Tribe and a Chapter relating to their "department", and area of expertise; Squads are formed to work on projects, then disbanded once the project is finished; anyone can join and participate in Guilds, which serve as interest/support groups for technologies/strategies/methodologies.
Edit: Corrected my brainfart. Thanks ssabev! Can't believe I did it twice...
In other words – you can't make a race horse by painting a pig brown.
Guilds - Informal interest groups (Linux, Python, agile development)
Never worked at Spotify but they seem very innovative (and they already let me rock out for eight hours everyday), so hey, nothing to bad say on my end.