It is necessary to understand the concept of bheda or distinction to understand the difference between Advaita Vedanta of Sri Sankara and Vishistadvaita Vedanta of Sri Ramanuja.
Distinction (Bheda)
In Hindu philosophy, such as in Vedanta, three kinds of distinction
(bheda) are recognized. They are as follows: (1) The distinction
between objects of the same kind, such as the distinction between one
cow and another cow. This is called sajatiya-bheda in Sanskrit.
(2) The distinction between objects of different kinds, such as the
distinction between a cow and a horse. This is called
vijatiya-bheda.
(3) The distinction between different parts of the same object, such
as the distinction between the tail and legs of the same cow. This is
called svagata-bheda.
Journey from Many to One essentials of Advaita Vedanta by Swami Bhaskarananda
Advaita Vedanta
The concept of Brahman in Advaita Vedanta has no distinction or bheda of any kind. This idea is called absolute identity. Brahman is not made of independent parts. So how does Advaita Vedanta explain the body-mind complex of Jiva and jagat given that Brahman has no inner distinction? It answers this question by claiming that the body-mind complex of Jiva and jagat have no ultimate existence and are dream like.
Vishistadvaita Vedanta.
The school of Qualified Non-dualism or Vishishtadvaita-vada, is a
school of theism. Its main exponent was Ramanuja. According to this
school, there are three ultimate realities – Ishvara (Saguna Brahman),
chit (jiva) and achit (Mother Nature or prakriti that evolves as this
material universe).
They exist in an inseparable relationship. Chit and achit, however,
are dependent upon Ishvara, who is independent. Chit and achit are
included in Ishvara as constituent parts. Such separation or
distinction within Ishvara is called svagata-bheda.
Ishvara is Vishnu. Ishvara is the repository of infinite virtues. He
is omnipotent, omniscient, self-existent, and of the nature of
consciousness. In relation to Ishvara, the jiva is like an atom (anu)
and is subservient to Him. Ishvara is both the efficient and material
cause of this world.
Upasana consists of ritualistic worship, devotional practices and
various kinds of meditation on God. The God-experience of an
individual is possible only through the grace of God. A person can
have moksha only after the person’s death. Moksha means living
blissfully in Vaikuntha (the abode of Vishnu) in spiritual bodies.
They acquire many divine powers such as omniscience, but unlike God
they cannot create, sustain, or dissolve the world. In spite of their
exalted state they remain subservient to God.
Journey from Many to One essentials of Advaita Vedanta by Swami Bhaskarananda
In Vishistadvaita Vedanta Ishvara is made of Narayana, Jiva (chit or conscious beings) and achit ( unconscious matter) and thus has svagata-bheda. Ishvara is like a fruit with three independent parts, seed, flesh and skin. This is the reason why Brahman here has organic unity (made up of independent parts) and not absolute identity. Jiva and jagat are real and not dream like. Brahman here is unity but qualified by distinction.
The universe and its living beings make up the body of God in Vishistadvaita Vedanta.
Universe and Brahman
You are the primordial deity with no origin. You are Prakrti, you are
Purusa, the protector of the world. You are Visnu the lord of the
universe. You are Brahma, with the universe for your body. You are the
first principle. O Visnu, you alone are the greatest luminary. You are
the supreme soul, O lord of Sri, you are the greatest abode. O lord of
the earth, Rudra enveloped by tamas originated from your fury. Brahma,
the creator of the universe enveloped by rajas was born of your grace.
The lord enveloped by sattva, was born of your grace. O Visnu, O
Rudra, you are identical with the universe.
Linga Purana I.36.4-8
Ishvara is said to be the efficient cause of the universe because the universe and its living beings do not have any existence apart from Him. The dependence is not mutual but one sided with the universe and its living beings being dependent on Ishvara and not the other way.