Endocrine glands are specialized structures that release hormones directly into the bloodstream. They are ductless and secrete a variety of substances that regulate metabolism, growth, development, and reproduction. Key characteristics include their ability to produce specific hormones, responsiveness to signals from the body, and their effect on target cells that have receptors for these hormones. Major examples of endocrine glands include the pituitary, thyroid, and adrenal glands. Their regulation is crucial for maintaining homeostasis within the body.