The document discusses the hematoxylin and eosin stain, which is the most widely used histological stain. It stains cell nuclei blue or black using hematoxylin, and stains cell cytoplasm and connective tissue fibers pink using eosin. The purpose of staining is to identify tissue structures and the presence or absence of disease. Common stains discussed include hematoxylin and eosin, Gram's method, Ziehl-Neelson's method, and Papanicolaou stain. The document also provides details on the chemistry and procedures for hematoxylin and eosin staining.