What is the informal organization?

"Up to this point, we have focused on formal organizational structures that can be seen in the boxes and lines of the organization chart. Yet many important relationships within an organization do not show up on an organization chart. Nevertheless, these relationships can affect the decisions and performance of employees at all levels of the organization. The network of connections and channels of communication based on the informal relationships of individuals inside the organization is known as the informal organization. Informal relationships can be between people at the same hierarchical level or between people at different levels and in different departments. Some connections are work-related, such as those formed among people who carpool or ride the same train to work. Others are based on nonwork commonalties such as belonging to the same church or health club or having children who attend the same school."