A gland is an organ or collection of cells that make a substance within the body. If the substance stays within the body, for example in the bloodstream, the gland is known as an endocrine gland. If the substance secreted passes out of the body, like sweat for instance, the gland is known as an exocrine gland.
Examples of glands include sweat glands, ovaries, testes, pancreas, thyroid and so on. Glands produce a wide range of substances including hormones (which regulate many body processes), mucus, sweat, tears and digestive juices.
Tumours which arise in glands are called adenocarcinomas.