This document discusses the different types of root resorption, including external root resorption. External root resorption is classified into external surface resorption, external inflammatory resorption, external replacement resorption, and external cervical resorption. External surface resorption is a self-limiting resorption caused by trauma or orthodontic treatment. External inflammatory resorption is often seen radiographically as an extensive lesion caused by necrotic pulp. External replacement resorption replaces the root surface with bone in a process called ankylosis. External cervical resorption is a localized resorptive lesion of the cervical area that may progress in an apical or coronal direction.