Warning: To not make it too long, I avoided quotes. In addition, this is merely a summary. Just like with Theravada, there are interpretations. Since I present things in a traditional way, please do not take it personal when I seem to despise so-called Hinayana tenets or practices.
We say that a Hinayana arhat still has limitations, obstacles that prevent him from achieving the welfare of others as well as his own. On the other hand, a [fully enlightened] buddha abandoned all obstacles and thus is able to achieve the welfare of others, and achieved his own. This is because there are two obscurations that a buddha has abandoned:
- Afflictive obscurations
- Knowledge obscurations
Since a Hinayana arhat has uprooted ignorance - the root of samsara - he has abandoned afflictive obscurations. This is because ignorance, anger, desire, etc. are afflictive obscurations, and once the root has been abandoned, all the other afflictions as well. However, a Hinayana arhat has not abandoned knowledge obscurations. Only a buddha has abandoned both obscurations.
There are four divisions of nirvana but I will speak of two. A Hinayana arhat achieved abiding nirvana. We also call it 'individual liberation' or 'solitary peace.' We say that this abiding nirvana is the extreme of peace. According to Asanga's Bodhisattvabhumi and other texts, after some time, the buddhas wake them up from their blissful individual liberation and tell them that more is to be done.
A buddha abides neither in the extreme of peace (abiding nirvana), nor the extreme of samsara. He achieved a non-abiding nirvana. He has abandoned the two obscurations and is, in this sense, beyond limitations. Therefore, he is able to benefit all sentient beings.
The way to abandon afflictive obscurations is by generating the wisdom of emptiness. Different Mahayna tenets posit emptiness differently, but they all posit the two selflessness: emptiness of person and emptiness of phenomena. As the Heart Sutra puts it:
Phenomena also are empty.
The way to abandon knowledge obscuration is by having the wisdom of emptiness conjoined with Bodhicitta. By generating spontaneous bodhicitta, one enters the Mahayana path of accumulation. On the Mahayana path of seeing, the wisdom of emptiness is conjoined with bodhicitta. When wisdom is imbued with bodhicitta, the mind becomes powerful and vast enough to abandon knowledge obscuration. We often refer to the following analogy:
The wisdom of emptiness is like an axe cutting through the two obscurations, while bodhicitta is a strong arm. Without bodhicitta, one's arm is not strong enough and can not abandon the two obscurations, thus achieving buddhahood. A Hinayana arhat has abandoned afflictive obscurations, but since he did not generate bodhicitta, he has not abandoned knowledge obscuration.
There are two divisions of knowledge obscurations:
- The imprints of ignorance, and
- The appearance of true existence that is caused by the imprints of ignorance.
Only a buddha is free from the appearance of true existence. Therefore, when a conventionality appears to a buddha, it does not appear together with the appearance of true existence and he realizes both truths simultaneously and with the very same mind: the omniscient mind of a buddha. On the other hand, whenever a conventionality appears to a sentient being (i.e. a non-buddha), it appears together with the appearance of true existence. Consequently, a sentient being cannot realize the two truths directly and simultaneously.
The omniscient mind of a buddha (the wisdom truth body dharmakaya):
- Directly realizes the two truths, simultaneously
- Is free from conceptualization, and unmistaken
- Knows all existent, past, present and future
In addition, according to Mahayana, a buddha obtained the four bodies (kayas). There are ways of dividing the kayas into two, three, four, or five but we generally speak of four. A buddha has a mental body that is seen only by Mahayana aryas and that teaches them only the Mahayana Dharma. It is called Enjoyment body (Sambhogakāya). A buddha has body that is seen even by ordinary beings and this is the Emanation body (Nirmanakaya). We say that Shakyamuni Buddha was a supreme emanation body because he taught he possessed the 32 marks and the 80 signs. A buddha has an omniscient mind that we call "wisdom truth body". It gives birth to the teachings that emanate from it, like the rays of the sun emanate from the sun. It is by emanating in this way that he benefits all sentient beings (even though not all sentient beings interact with his teachings, but that is another subject of debate.)