I'm often interested in the history of Buddhist texts and being a Theravadan i'm not very familiar with the Mahayana texts. Please help to educate me in this manner.
While there is no commonly recognized single oldest Mahayana sutra (see Wikipedia for general treatment of the topic), many scholars and practitioners believe some of the Prajnaparamita-series sutras to be very old.
Specifically, Aṣṭasāhasrikā Prajñāpāramitā Sūtra ("Perfection of Wisdom in 8,000 Lines") is believed to be written as early as 1st century BCE -- the earliest date I have personally seen anywhere for a Mahayana sutra. This sutra exists in two versions, one in verse and one in prose. As per Edward Conze, "for early Mahayana sutras that was quite a normal procedure."
The verse version is known as Prajnaparamita-Ratnagunasamcayagatha or simply "Ratnaguna". Based on linguistic analysis, Ratnaguna is thought to be the earlier of the two. According to Edward Conze, Ratnaguna's first 41 stances, known separately under the name of "Arya Prajna-Paramita Carya Gatha", are the earliest core. This makes Arya Prajna-Paramita Carya Gatha a good contender for a honorary title of the earliest Mahayana sutra.
It is available here as part of Edward Conze's complete translation of Ratnaguna and here translated by a committee under guidance of 16th Karmapa. Here I am posting my amateurish translation of the first 8 verses from BHS:
That's quite a broad question, I certainly can't give it justice, but here are some starter points while we wait for better answers:
Mahayana Texts are preserved either via the Chinese Canon or the Tibetan Canon and certain of them were probably written outside of India.
These text were not memorized, these almost certainly were composed-- they are just too long to have been memorized.
They are in a completely different style-- attempting to imbue a sense of awe and wonder. I can't say it always works for me, some texts can get rather tedious in their "better than infinity plus one" exaggerations.
They tend to follow a pattern-- it starts out describing the Buddha getting ready to deliver a teaching, describes who is in attendance, and so on. The often include large sections about the merits of making copies of the sutras.
Thematically, I definitely don't have time to do them justice, they cover a lot of different themes. Some describe mantras and their benefits. Some describe Bodhisattva's-- their previous lives as monks and the various heroic vows they made. Some of them are philosophical discussions, sort of like Plato's dialogs.
The tantras sometimes get included in the broader category of Mahayana texts, but I haven't tried to read the tantras yet.
Keeping dependent arising in mind. The Gandharan Buddhist texts according to scholars contain samples of ancient Mahayana texts and teachings from the Dharmaguptaka school, the discovered texts have been dated to the 1st century CE. "Whose Buddhism in Truest?" an article written by Linda Heuman in Tricycle Magazine provides insightful information regarding these early texts and possible ramifications regarding the development of Buddhist traditions.