One of the most common answers to the question "What is the difference between Theravada and Mahayana?" is the answer "The Bodhisattva concept." Mahayana teaches that one should not try to become enlightened as soon as possible, as Theravada advises, but should strive to become a fully enlightened Buddha, or even postpone enlightenment indefinitely until all beings are liberated from the wheel of Samsara. This answer with regard to the difference, however, is not 100% accurate. There are other differences too of course---for example Mahayana metaphysics are rather more sophisticated and, I think, more profound---but Theravada has a Bodhisattva concept also, or rather a Bodhisatta concept.
Before going any further in that regard I'd like to discuss briefly what the term Bodhisatta means. The term bodhisattva was translated from bodhisatta in Pali, or perhaps more likely a Buddhist Prakrit language very similar to Pali. The term is a combination of two words, bodhi and satta; the first part is the same in Pali and Sanskrit and means "awakening" or "enlightenment." The second part, satta, is not so certain. In Sanskrit the word sattva means "creature" or "being"; thus Bodhisattva means "Enlightenment Being." But this is not necessarily what the word originally meant. It does seem a little odd that a being who is not yet enlightened, and in Mahayana Buddhism may not even be trying to become enlightened any time sooner than the end of the Universe, would be called an Enlightenment Being. The word satta in Pali has more than one possible meaning, and its rendering into Sanskrit as sattva could possibly be a mistranslation.
Mistranslations from an older language like Pali into Buddhist Sanskrit have happened before. For example, in the doctrine of the Two Truths, ultimate and conventional, the very ancient term sammuti---"convention, consent, tradition"---somehow got converted into the Sanskrit word saṁvṛti, which means "covering" or "concealing." Thus from sammuti sacca, conventional truth, arose saṁvṛti satya, concealing truth (although the philosophical implications are practically the same).
Getting back to satta, however, in Pali it can also mean "seven" (in Sanskrit, sapta), but I think we can easily rule out "Enlightenment Seven."
Also, satta can be a past participle of the verb sapati, "to curse," but "Enlightenment-Cursed" also appears very unlikely.
Yet satta in Pali may also be a past participle of the verb sajjati, forming an adjectival noun. Sajjati means "to be attached to," "to be hung on to." Thus the original meaning of the word bodhisatta may have been "One who is committed to enlightenment" or "One who is bound for enlightenment." This would seem to make more sense, and is apparently more in harmony with the older Theravadin conception of the idea.
In orthodox Theravadin tradition a bodhisatta is indeed one bound for enlightenment, or rather bound for full-blown Buddhahood. He is one who is destined to become a fully enlightened Buddha, one who rediscovers Dhamma after it has been lost to the world, and then teaches it, thereby reintroducing Dhamma (and Buddhism) into the world. He reaches this state by bringing various qualities called pāramī, including generosity, truthfulness, loving-kindness, and so on, to perfection over a period of incalculable eons, lifetime after lifetime. According to the commentarial literature there are certain other requirements for a true bodhisatta: for example he should already have a mastery of jhāna, or deep contemplative states, and he must also make a vow to become a Buddha in the presence of one who already is a Buddha---and receive a prophecy from that Buddha assuring him that he will ultimately succeed. "Our" Buddha Gotama reportedly received his prophesy from a Buddha named Dīpaṅkara more than 2 incalculable eons in the past, on a different world.
Looking at the most ancient texts available, it seems likely that originally there was no difference between Buddhas and ordinarily enlightened Arahants. "Buddha" literally means awakened or enlightened, and Arahants are certainly that, otherwise they wouldn't be Arahants. And the Buddha himself is sometimes called an Arahant, as in the famous formula namo tassa bhāgavato arahato sammāsambuddhassa---Reverence to him, the Blessed One, the Worthy One (Arahant), the Truly, Fully Enlightened One. I feel that in all probability the Buddha, like Jesus also, was saying essentially, "Look, I'm a human being like you, and I did it. If I can do it, you can do it too. You can be like me."
However, it is human nature to glorify great leaders, political, military, cultural, or spiritual. So as Buddhism developed into a popular religion followed largely by simple and not very spiritually talented folks, the Buddha became more and more glorified, and eventually even worshipped. For many this worship replaced the way of life taught by the master, as also happened with Christianity. Jesus became the Son of God, and Buddha became higher than any god, including the Mahā-Brahmā, who allegedly descended to earth to worship Buddha. Sakka the King of Gods reportedly became one of the Buddha's chief followers.
The Buddha's glorification involved an ancient genre of literature called Jātaka, which described the Buddha's former lives as a Bodhisatta, often in moral tales portraying talking animals, similar to the fables of Aesop. Many of these stories were common Indian legends which were appropriated and modified by the Buddhists. Anyhow, the idea developed that the Buddha heroically strived for eons to perfect himself to the point of Buddhahood, sometimes as an animal, sometimes even in hell…which implied that he was somehow more noble, more wise, and more enlightened than even his most successful disciples. Thus the discrepancy between an "ordinary" enlightened being and the extraordinary Buddha; and thus also the genesis of the Bodhisatta/Bodhisattva concept.
I am no expert on Mahayana Buddhism, but I have been led to understand that Mahayana Buddhists are supposed to shun ordinary enlightenment as something selfish and perhaps even cowardly (running away from Samsara), and to strive to become fully enlightened Buddhas---or even to forgo enlightenment altogether until all other beings in the Universe have also been liberated. One of the four main vows made by Mahayanist Bodhisattvas is, "Beings are numberless: I vow to save them."
It seems to me, though, that this idea involves some faulty logic. According to Buddhist philosophy, this Universe (or, if you prefer, this endless series of Universes) has no creator and no beginning; so it has existed already for an infinite length of time, and there are still an infinite number of suffering, unenlightened beings. This would seem to suggest that there will always be an infinite number of suffering, unenlightened beings. If an infinite length of time has already passed and the number of suffering beings is still numberless...
It may be that the vow mentioned above is a noble gesture of rejecting enlightenment since everyone can't have it. It would, however, seem to be practically the exact opposite of what Gotama Buddha reportedly taught, as indicated by the oldest Buddhist texts we have. The time for enlightenment is Now.
There is another approach to the apparent dilemma which I came across in a profound Chinese Zen text called the Sutra of Hui Neng, also called the Platform Sutra.* It has been a long time since I read it, but if I remember correctly Hui Neng, the Sixth Chinese Patriarch of Zen, says essentially that once you become enlightened you look around and see that everyone else is enlightened too. In other words, when you see with perfect eyes all that you see is perfect. "To the pure man all things are pure." As Benedict Spinoza said in his Ethics, "By reality and perfection I mean the same thing." This world of suffering is a creation of our own delusion. Consequently, the only way to liberate all beings would be to liberate ourselves; then we see that, ultimately, there are no beings to liberate.
There is another way of looking at the issue: I have found that the directions for Bodhisattvas in Mahayana texts like the Diamond Cutter Sutra are very similar in details to descriptions of the psychology of a sage in ancient Theravada texts like the Sutta Nipāta. It could be that once one realizes a certain stage of wisdom, labels such as "Arahant" and "Bodhisattva" no longer stick. One has gone off the scale, and quibbles about what label to apply become irrelevant.
The Bodhisattva Avalokiteshvara,
the earlier (male) version of Kuan Yin
* I used to think that the Sutra of Hui Neng was, finally, a Buddhist text whose authenticity was relatively certain, as it is reportedly a verbatim transcript of a Dharma talk given by the Patriarch, recorded by a professional scribe who attended the talk. I figured one couldn't be more trustworthy than that, and I trusted this for years. Then I came across a different translation at a library in Rangoon, written by a fellow named Yampolsky, of Columbia University. In his introduction he made a number of very interesting and disillusioning statements. First he declared that his translation was based on a recently discovered manuscript of the Sutra which is older than the traditionally accepted version and differs from it in many respects. Then he went on to deduce that the Sutra doesn't really represent a discourse of Hui Neng, but is a forgery written by one of the Patriarch's disciples, which then underwent extensive modification over many years…although it was in all likelihood originally based on the teachings of the master. Not only that, but he further wrecked the illusion by hypothesizing that Hui Neng probably wasn't really the Sixth Patriarch at all, and that the story of his receiving the emblems of the Patriarchate secretly from the Fifth Patriarch, and so on, are apocryphal. The real Sixth Patriarch was presumably the timid senior monk belittled in the story, who wrote inferior poetry on the monastery wall; but his school, "the gradual school" of the north, died out, so no one could contest the claims of the forged Sutra declaring that Hui Neng was secretly the Sixth Patriarch. This would explain, among other things, why the lineage ended with the Sixth, with no Seventh. It all goes to show the hopeless unreliability of "authentic" sacred scriptures, and the unscrupulousness even of dedicated spiritual practitioners when it comes to recording them. The moral of the story is that the effect of a text---its usefulness in expanding consciousness, or at least in knocking us out of ruts---is of real importance, not its supposed authenticity or even its factuality.