Introductory abstract:
Brahman is considered to be the ultimate reality in this cosmological affair in the philosophical outlook of the Indian mindset. It is the primary point in Advait Vedanta school which is based on the vedanta school which is etymologically meaning the “literal end of vedas” and these are the teachings of and crux of the vedas. Shankara believed in the nondualistic nature of brahman and atman and a unity between them. Then Shankara’s idea was challenged by Ramanujacharya ji who asserted that brahman in unity is having two essential forms which are, matter and spirit, his idea was vishtha advait school. Let us understand this aspect.
Main Idea: Shankara’s Idea on Brahman
Brahman is absolute, supreme, non-dual and ultimate reality for shankara. He believed that it is beyond all explanations because no explanation or descriptionless could complete, therefore, discriptionless. It is the fact that everything existing, has originated, subsists and merges in the brahman and it is self-luminous and self explanatory. It is beyond time and space, it beyond speech and mind as well. Brahman is satchittaanand in the sense that sat: Indubitable, chitta: consciousness and anand: bliss; therefore, it is pure existence, pure consciousness and pure bliss. 
It is explained as “Nirguna” which means not the fact that it has no qualities, but it is beyond all qualities. This is because of the fact that if we assert qualities, it is accompanied with negation, as every assertion is negation but brahman is beyond all this; hence it is best explained as “neti…neti” which means as per shankara, not this…not this. Because every assertion is having contradiction but brahman of shankara is contradictionless as well. It is, therefore, one, partless and distinctionless. Shankara, succinctly explains that, due to maya illusion and ignorance the individual jiva atma views the world as dual in the sense of jagat and brahman and believes itself to be different from brahman. Additionally, to further his expression, he asserts three different realities which are as follows:
(1) imaginative reality: pratibhashika satta, (2) empirical reality: vyavaharika satta, (3) Transcendental reality: paramarthika satta; it could be easily explained by hindu parable of example of snake-rope and dream state; when we believe that it a snake and not a rope, when we are in dreams- this is accepted in the paribhashika satta but is refused and rejected in vyavaharika satta. The waking up from a dream, believing the rope is actually a rope and not snake- it’s the part of vyavaharika satta which is more durable, however, it is refuted at the transcendental state. But the paramarthika satta is the only reality which is irrefutable and therefore, brahman is the ultimate noncontradicted reality.
Shankara rejects the use of pramana of sensation, perception, and inference in knowing the brahman. It is, as per shankaracharya ji, devoid of differences; the fact that there is no reality alike brahman (swajatiya bheda), there is no reality other than brahman (vijatiya bheda) and that it is devoid of internal (swagat) differences.
Criticism by Ramanujacharya:
Ramanuja believed that “creation is real ” unlike shankara. He supported “brahma parinamavada from satkaryavada theory of causation” wherein the cause changes itself into the effect. Brahman, as per ramanuja, is composed of both chit and achit which are co-eternal with brahman. He has actively and bitterly criticized the fact that if we will hold the brahman to be non-dual, then the diversity of this world will be left unexplained. Another epistemological issue which arises with this theory of shankara is that, ramanuja is presupposing knowledge to identity of differentiation and discrimination. The pure identity cannot be an object, he argues, and since Shankara holds that brahman is pure identity, knowledge is not possible. Ramanuja has criticized shankara’s theory of maya, wherein, he has used the sapthaanupapatti (7 charges) against maya such as the swaroopanupapatti and anirvachineyanupapatti which tells that it is plainly self contradictory to explain that brahman cannot be explained.
Ramanuja’s Solution:
Brahman and isvara are same for him. He says brahman becomes a cause and effect in such a way that, during the pralay- dissolution, brahman becomes the cause and all the unembodied souls with the gross matter becomes its body; during the creation, brahman becomes the effect in the way that unembodied soul becomes embodied souls as per their karmas, the gross matter becomes subtle matter and in this, the creation of the world is manifested. In the first instance, it is called karyavastha and the second man infestation is known as the karyavastha. 
Conclusion:
It could be fairly concluded that ignorance and avidya, we see ourselves different from brahman and maya conceals the ultimate real truth and superimposes the qualities on the brahman and as the result it is saguna brahman– it is the soul of souls, soul of nature and object of worship. It is called the satchittananda. Brahman conditioned by maya is isvara, qualified brahman is isvara and isvara is the personal aspect of the impersonal brahman. Dasgupta, provides the same analogy from the vedantic school parable of shepherd in the character of king, ruler and conqueror; it tells about the “accidental qualities: tathasatt gunas”. In conclusion we can say that, for shankara, brahman and isvara is different, brahman has no qualities but for ramanuja, the brahman and isvara is same and brahman is having certain qualities.
References:
S.N Dasgupta; History of Indian Philosophy (19th century)
