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What is the constitution of the endocrine system?

The endocrine system is constituted by the endocrine glands.

What is the difference between endocrine gland and exocrine gland?

Endocrine gland is a gland whose secretions (called hormones) are collected by the blood and reach the tissues through the circulation.  The hypophysis and the adrenals are examples of endocrine glands. Exocrine gland is a gland whose secretions are released externally through ducts (into the skin, intestinal lumen, mouth, etc.). The sebaceous glands and the salivary glands are examples of exocrine glands.

What is the histological nature of the glands? How are they formed?

The glands are epithelial tissues. They are made of epithelium that during the embryonic development invaginated into other tissues.

In the exocrine glands the invagination have preserved secretion ducts. In the endocrine glands the invagination is complete and there are no secretion ducts.