Final answer:
Glands are derived from epithelial tissues and can be classified by their secretion methods and whether they release products through ducts or directly into body fluids.
Step-by-step explanation:
Glands are secretory tissues and organs that are derived from epithelial tissues. There are multiple types of glands, including exocrine glands which release their products through ducts, and endocrine glands which secrete hormones directly into the interstitial fluid and bloodstream. Glands are also classified by their secretion method, with merocrine glands releasing products as synthesized, apocrine glands pinching off the apical portion of the cell, and holocrine glands whose cells rupture to release their contents.