A studio is an office of game developers located in a geographical area.
For small companies, the terms studio, team and developer are all interchangeable. However for larger companies the distinctions start to matter. A big publisher like Electronic Arts, for example, has many studios all around the world and those studios contain multiple teams working on different games.
Studios often try to maintain distinctive identities from their owners. Blizzard or Bioware, for example, are two formerly independent game developers who became studios of Activision and Electronic Arts. It often helps promote a game with fans to preserve that sense of creative authenticity, as it feels more human than just having one corporate brand. However in some other cases (particularly with Japanese publishers), the opposite is true. Nintendo's individual teams are not well known, nor are Capcom or Konami's.
Studios are usually run by a studio head, a very senior executive producer.