A gland a set of epithelial cells specialized in the production and secretion of chemical substances. There are also unicellular glands, in which a single cell works alone to secrete substances.
The endocrine glands secrete the chemicals they produce directly into the blood or lymph. These chemicals are hormones that in small amounts can produce multiple effects in organs far from where they originated.
The exocrine glands release their products into ducts that carry the substance to the place where it will fulfill its function. This place can be inside or outside the body.
The mixed glands produce substances that dump into the blood and other substances that dump into ducts, so they work with endocrine and exocrine.
Example of endocrine glands : the islets of Langerhans that produce the hormone insulin that regulates glucose metabolism.
Example of exocrine glands : sweat glands, some of which are distributed throughout the body, opening directly to the outside through the skin; while others are associated with areas where there are many hair follicles (groin, scalp and armpits) through which they reach the surface of the skin.
Example of mixed glands : If we consider the entire pancreas as one gland, we can say that it is a mixed gland. On the one hand, Langerhans cells act by producing hormones (endocrine secretion) and on the other hand, other cells of the pancreas produce enzymes such as trypsin, amylase, and lipases (exocrine secretion).