The document discusses different types of communication structures in organizations, including formal and informal communication channels. Formal communication follows the organizational hierarchy and approved lines of communication, while informal communication occurs through unofficial social interactions. The key types of formal communication structures described are the chain, Y-shape, wheel, and circle networks. Informal communication spreads through the 'grapevine' and allows for faster sharing of information across departments in comparison to formal channels. However, the accuracy of information transmitted informally cannot be guaranteed."