A Developer is an individual, or a company that commits resources towards the development of a software title. This resource can be practically anything, from raw code, to overseeing compiling of code, or quality assurance of the finished article.
For the development of games, the developer is normally a Games Studio, usually part of a larger corporate company. For example, Simcity 4 is developed by Maxis; however, Maxis is wholly owned by Electronic Arts, who also serve as the game's publisher.