Exocrine Glands: These are glands that produce and secrete substances onto an epithelial surface by way of a duct.
Endocrine Glands: These are ductless glands that produce hormones which they release into the bloodstream.
Islets of Langerhans: Clusters of cells within the pancreas where insulin and glucagon are produced; they include alpha cells (which produce glucagon) and beta cells (which produce insulin).