Complex sentence: A disk image file is usually created by creating a sector-by-sector copy of the source media, ignoring its file system, and thereby perfectly replicating the structure and contents of a storage device.
Split into simpler sentences: A disk image file is usually created based upon the sectors on the medium (bit copy), ignoring its file system. As such, a disk image contains all the information necessary to perfectly replicate the structure and contents of a storage device.