The document defines and classifies glands. It discusses that glands are aggregations of cells that produce secretions or excretions. Glands are classified as exocrine if they release their products onto epithelial surfaces through ducts, or endocrine if they release into the bloodstream. Exocrine glands are further classified based on their duct morphology and shape of secreting portions. The document also discusses the histological structure of exocrine glands.