This document provides an overview of virtual world systems and technologies for developing virtual worlds. It discusses key features of virtual worlds like social interaction, large persistent environments. It then examines some successful 2D and 3D virtual worlds that break conventions. The document stresses that successful development requires balancing the design, content, and technology. It also advises working within time and budget constraints. Finally, it surveys various 2D and 3D virtual world technologies and considerations for choosing a technology.