The Default Mode Network is best described as the brain state for when a person is not focused on external world, and instead brain it at wakeful rest. Think daydreaming and mind wandering. Primary brain structures involved are Medial Prefrontal Cortex, Posterior Cingulate Cortex, Precuneus, and Angular Gyrus.
Interesting Cytoarchitecture
From Yana Log Notes, interesting lectures from Casey Paquola.
In essence, the Default Mode Network is not simply a “task negative” network ( it was originally assumed to be so due to inactivity during some “goal oriented” tasks ). It also recieves significant activity during memory related tasks, social cognition, and others. Those all depend on integration of internal and external information. That is a primary function of the Default Mode Network, as an association area.