The Supreme Personality of Godhead, Kṛṣṇa, is the fountainhead of all incarnations. Lord Balarāma is His second body.
These two are one and the same identity. They differ only in form. Lord Balarāma is the first bodily expansion of Kṛṣṇa, and He assists in Lord Kṛṣṇa’s transcendental pastimes.
That original Lord Kṛṣṇa appeared in Navadvīpa as Lord Caitanya, and Balarāma appeared with Him as Lord Nityānanda.
May Śrī Nityānanda Rāma be the object of my constant remembrance. Sańkarṣaṇa, Śeṣa Nāga and the Viṣṇus who lie on the Kāraṇa Ocean, Garbha Ocean and ocean of milk are His plenary portions and the portions of His plenary portions.
Lord Balarāma is the original Sańkarṣaṇa. He assumes five other forms to serve LordKṛṣṇa.
He Himself helps in the pastimes of Lord Kṛṣṇa, and He does the work of creation in four other forms.
He executes the orders of Lord Kṛṣṇa in the work of creation, and in the form of Lord Śeṣa He serves Kṛṣṇa in various ways.
In all the forms He tastes the transcendental bliss of serving Kṛṣṇa. That same Balarāma is Lord Nityānanda, the companion of Lord Gaurasundara.
( cc.adi.5. 4-11)