There are scholars who say that the differences is not that great because the editor of the Pali Cannon is said to have been a Mahayana buddhist. It is said that he subtly introduced certain nuances that made it coherent from a Mahayana point of view. I don’t know if it is true. My own teacher Alfonso Verdu wrote a book showing that everything in Mahayana Buddhism can be traced back to the original Hinayana scriptural tradition in Pali. So whether the Mahayana was infused later or whether Mahayana was there from the beginning. The fact is that the difference between Mahayana and Hinayana is not as stark as it might seem.
My own view is this. Buddhism and Jainism (http://en.wikipedia.org/wiki/Jainism) were both similar heresies which offered escape from Karma by realizing Nirvana. Jainism was older than Buddhism by about three centuries so we can see it as preparing the way for Buddhism. They were both non-dual in character but Buddhism was more radical because it gave up on Being. I am still not clear whether this is true of Jainism. It appears from what I have read that Jainism did not reject Being but had another idea established in the seven principles. But it is clear that the seven principles are an approach to the nondual similar but subtly different from the tetralemma. So I believe that Mahayana is a combination of Buddhism and Jainism at the theoretical level plus other elements that make Mahayana Buddhism more logical, for instance the recognition of the contradiction of Nirvana as a goal and the substitution of the Bodhisattva ideal. Or getting rid of the split between dharmas that are real and those that are empty. Mahayana is vast but it is characterized by the push toward consistency in the belief system with a refocus on compassion as the key characteristic that is emphasized over personal spiritual ends.
In Jainism there is the seven principles.
“Syādvāda provides Jains with a systematic methodology to explore the real nature of reality and consider the problem in a non-violent way from different perspectives. This process ensures that each statement is expressed from seven different conditional and relative viewpoints or propositions, and thus it is known as theory of conditioned predication. These seven propositions are described as follows:”
- 1.Syād-asti — “in some ways it is”
- 2.Syād-nāsti — “in some ways it is not”
- 3.Syād-asti-nāsti — “in some ways it is and it is not”
- 4.Syād-asti-avaktavya — “in some ways it is and it is indescribable”
- 5.Syād-nāsti-avaktavya — “in some ways it is not and it is indescribable”
- 6.Syād-asti-nāsti-avaktavya — “in some ways it is, it is not and it is indescribable”
- 7.Syād-avaktavya — “in some ways it is indescribable”
We can see this as a proto-tetralemma that does not yet leave Being for Existence as the Tetralemma does. It introduces a third state which is not describable rather than existence without Being.
The tetralemma shortens this to A, ~A, Both, Neither and then points in silence toward emptiness which is outside this logical set. If something is indescribable that means it cannot be expressed in language, and therefore one would not be using Being in ones description of phenomena, so saying that something is indescribable is a way to avoid Being, without actually giving it up. The key point is that the seven principles are logically exhaustive, they point to what exists that cannot be described, and thus is not attached to Being, and it produces a third state which could later be seen as existence which is a separate stance toward phenomena created by the Buddhists as the locus of the undescibeable outside of Being, especially the Being of the self. However, in Hinayana the indescribable existence as a nondual standing beyond Being is only attributed to the self. Other dharmas are accepted as having Being and thus reality. Dharmas are really tattvas (small independent mechanisms that function to support life very much like the Sumerian Me, which is also the copula in Sumerian.
Because the Buddha taught for 40 years we can be reasonably assured that what is in the Pali Cannon is pretty accurate as to his teaching. And it is pretty focused on things like the meditation on corpses as ways to realize the reality of impermanence in life. Actually watching the corpse decompose is a sure way to know that it is really an aggregate that disperses. Buddhism was similar to the philosophy of Heraclitus, believing that everything is flux of aggregates in continual change with no stability. However, interestingly Hegel sees in his Logic the Buddhist concept of emptiness (nothing) as the antithesis of Being, and he recognizes that the philosophy of Heraclitus is the philosophy of Becoming which is the synthesis of Buddhist nothingness and Being. Thus of all philosophers in the Western Tradition, Hegel is the one to recognize that the fundamental opposition is between Being and Existence (as nothing, as emptiness). And it was recognized by the Arab philosophers that Being had some residue of Wajud in it, but that it was much more than what is implied by existence in Arabic, so they coined the term Kun (making) which then become existence when retranslated back into Latin. Of course, no one took much notice of this core of existence within being until the Existentialists came along and wanted to create a fundamental reversal in the tradition by saying that existence precedes essence, which is obviously true because you have to exist before you can be something of a particular kind from a purely logical point of view. But everything that exists is already of a particular kind so it never was an issue, until there was a desire for an alternative to Being, with all its confusions, contradictions, paradoxes, and absurdities. Existence suddenly focuses us on the individual and its viability, and leads in Biology to an Autopoietic orientation (Maturana and Varella) rather than a species centric orientation.
So Hinayana Buddhism is the original form of Buddhism probably almost exacty as taught by the Buddha and established in the Pali Cannon. Mahayana is rooted more in Sanscrit and takes up a position against various Hindu philosophies and strives for coherency so that its arguments for its point of view can be taken seriously. These are of course very general characterizations, because there are many schools of both vehicles, but I think more of Mahayana because this later school was willing to go with new Suttras beyond the Pali Cannon in which the Buddha is seen saying more and more interesting things as time goes on.
My recommendation if you want more detail is that you read the History of Buddhist Philosophy by David J Kalupahana: