I wrote something about that in another place in the forum. Here it is:
Around 300 CE the Yogacara school (also called ´Vijnanavada´ = teaching of consciousness) was founded by Maitreyanatha and elaborated by Asanga and Vasubandhu, placing ´consciousness´ (vijnana) and ´mind´ (citta) in the center of its theory. This school went into competition with the older Mahayana school called ´Madhyamika´ (Middle Path), founded by Nagarjuna in the 2nd century CE, which gave ´emptiness´ (shunyata) a central theoretical position.
Common to both schools was their goal of qualifying their followers to obtain Buddhahood, differing only in the theoretical approach. While the Madhyamikas thought ´emptiness´ to be an optimal conceptional beacon for truth-seeking trainees, the Yogacaras considered ´consciousness´ to be the better choice.
As to phenomena, for example, early Buddhism takes them for really existing but transitory and causing pain, what corresponds to Buddha´s dualistic view on the world, dividing it into two spheres of reality, the positive and perfect ´Nirvana´ and the negative and imperfect ´Samsara´. The Mahayana, however, transcends this dualism by uniting all aspects of reality in all-embracing nondualistic concepts, e.g. ´emptiness´ and ´mind´/´consciousness´.
For the Madhyamikas, phenomena are ´empty´ in the sense of having no inherent self-nature, that is, no reality. Hence the Nirvana and the world of phenomena, the Samsara, are both empty of self-nature, from what follows that they are identical.
For the Yogacaras, phenomena are unreal and illusionary, too, but they are mental constructions, produced by the absolutely real ´citta´ (mind) resp. ´vijnana´ (consciousness), both concepts being coextensive. Individual consciousness is founded in sort of a universal consciousness called ´alaya-vijnana´ (storehouse-consciousness) which is largely unconscious to non-enlightened people. The relation between the Alaya and conscious mental activities can be analogized by the picture of an ocean (Alaya) and the waves on its surface which represent the activities.
Here we see Sigmund Freud´s theory being anticipated by almost two millennia, of course with the difference that the Freudian unconscious is individual and the Alaya-vijnana universal. However, there is a Freudian aspect also in the Yogacara concept insofar as the mentioned ´waves´ on the surface of the Alaya are effects of unconscious previous ´seeds´. These seeds were produced by the subject´s experiences and actions in the past and then sunken into the Alaya; from there (that is, from a kind of individual unconscious) they influence the way the subject is feeling and acting in the present (what corresponds to the concept of Karma).
In short: the waves are individual but the Alaya, from where the waves emerge, is universal.
So, while the Madhyamikas consider ´emptiness´ to best describe the ultimate reality, the Yogacaras for that purpose favor the concept of consciousness resp. mind (what is expressed in the Yogacara concept of ´cittamatra´ = mind-only).
Of course, both schools mutually criticized their approaches. In the Madhyamikas´ view, the Yogaraca idea of consciousness as the only reality means to ascribe a kind of substance (mind) to reality, what appears to completely contradict the Madhyamikas´ teaching of emptiness = non-substantiality. The Yogacaras, on the other hand, considered the concept of ´emptiness´ to be top-heavy and to tend towards nihilism, that is, to describe reality as consisting of nothing, what appears illogical to the Yogacaras since enlightenment shows the ultimate reality to be anything but empty, moreover, without consciousness this reality could not be experienced at all. It goes without saying that both parties knew that their disagreement concerned only the means of theoretical approach to the ultimate reality, on the empirical nature of which they did not disagree, in the sense that there is only one Rome but many paths to it.
Here are some quotes from one of the main texts of the Yogacara school, the ´Lankavatara Sutra´:
Suchness, emptiness, the limit, Nirvana, Dharmadhatu, variety of will-bodies - they are nothing but Mind, I say.
What appears to be external does not exist in reality, it is indeed Mind that is seen as multiplicity; the body, property, and abode - all these, I say, are nothing but Mind.
Free from the faults of the philosophers and Pratyekabuddhas and Sravakas is the truth of the inmost consciousness, immaculate, and culminating in the stage of Buddhahood.
The Buddha, together with the sons of the Buddha and the wise men, accepting the offerings, discoursed on the truth which is the state of consciousness realised in the inmost self.
´Self-nature´ or alternatively ´own-being´ correspond to the Sanskrit term ´svabhava´ what refers to the supposed substantial existence of an entity, or in other words, to the supposed essence of an entity. ´Substantial´ means that an entity is existing largely independent from its environment, having an indivisible core of identity that makes it unique. The conventional conception of ´reality´ implies that reality is composed of such substantial entities.
For example, this big guy ´is´ Sheldon and that bang girl ´is´ Penny. However, in the Buddhist view Sheldon as well as Penny are materially composed of transitory material dharmas and mentally composed of transitory mental dharmas, thus being dissoluble into components without any essence. This is true for every entity: they are like onions without a core (of identity). In the human sphere, Buddhism calls this ´an-atman´ (non-self), what of course is already known to you. The said components of an entity are called ´dharmas´.
To the Theravada school, these dharmas are ´real´ as already mentioned by me when referring to “early Buddhism”. However, the Mahayana goes a step further by denying even the existence or reality of dharmas, the Madhyamikas by deeming them ´empty´ and the Yogacaras by deeming them to be mere mental constructions out of the universal mind (alaya vijnana) which is, in the Yogacara view, the only reality.
Again, these are very basic introduction to Yogacara. Some mistakes are repeated (which is normal, as even professional philosophers sometimes gets parts of Yogacara wrong; Xiong Shili being one of them). It left out the three type of self nature I mentioned earlier; which is absolutely crucial to understand Cittamatra.
Yogacara talks a lot more than just "the mind is real". The philosophy is an offshoot from the Sarvastivadin and the Sautrāntika, mixing in Mahasamgika (Madhyamika predecessor) concepts. Sarvastivadin considers all dharmas as real, and Yogacara, as an offshoot, classified and addressed all the dharmas from all these schools, and also affirms certain dharmas as "self nature divorced from concepts". It's important to note that while Madhyamika doesn't deny the external world like Yogacara does, it is Madhyamika which considers all phenomenon as illusionary. Yogacara does not. Mind in Yogacara does not simply mean the conceptual world (the conceptual world merely forms the sixth conciousness of thoughts, Yogacara proposes the seventh in Mana, and the eighth in the storehouse Alaya consciousness). Concepts merely forms the "self nature of clinging", the first of the three types of existence Yogacara classifies things into.
From the perspective of conventional truth, the "self nature of clinging" is pure concepts and illusionary, but there are dharmas in this world which are real (forming the second type of classification; self nature of co-arising). Sound is real, the colors red, blue, white, and yellow are real dharmas. They are not mere concepts. However, they are not physical entities and only exist relative to the subject perceiving them. They are also conditioned, hence impermanent, but exist as something real nonetheless.
The third type of existence is the real mind (when viewed through the tainted lense this real mind becomes "self nature of co-arising"). But note that this mind is not a first cause like an universal atman. The alaya consciousness isn't a self. It isn't inherent in an individual, ready to be tapped into when you look for it deeper. Rather, like the Sarvastivadin Nibbana, the real Tatha is only manifested after the delusion of the self is annihilated and viewed as empty. Conventionally speaking, its better to view your self (Mana) as a delusional objectification of the Alaya (which is the component of all the seeds (conditions for dharmas to manifest) stored); and hence your self (including your concepts) is merely a projection of the real Alaya.
There are still debates over whether the Alaya consciousness is individual based or universal. But ultimately speaking, its neither one nor many, because its very nature is also empty. Yogacara does not deny emptiness, it merely considers some empty things to be real and existent, whereas Madhyamika considers them to be illusionary and fake.