I am concerned that Mahayana has been sold short; Zen even holds certain aspects of mantrayana/tantrayana as well. There appears to be a remnant of secularism ringing in my ears.

If I recall correctly, Buddhism was established in far east Asia before Songtsen called for its establishment in Tibet. Recent archaeological findings along the Silk Road confirm that Tibet was well aware of this, yet sent to India for teachers, teachings, and texts leaving a perfectly able, completely qualified, and fully willing, swarthy, furry Buddha Dharma to stare at a wall for nine years).

The Inconceivable with the Eight
Are a fish with a bicycle
Brown M&M's, Blue M&M's
Chikpa, chik khyang

Taste like Dharma
Smell like Dharma
Sound like Dharma
Look like Dharma

Sit and Walk like Dharma
Dream and Wake like Dharma
It is probably Dharma
Probably perfectly good Dharma

Buddha Dharma in any culture
Is Gone, Gone Far Beyond
Unlike the internet
Suchness is unbreakable

This is basically how I understand it, the Dharmakaya - the Dharma Body, is basically the Dharma, the Law, the Truth, The Teachings of all of existence and non existence, containing both side of the Four Noble Truths - of Suffering and the End of Suffering. The sentient beings in the Dharma as reflected by the physical body of the Buddha as well all of us, are the manifestation of this Dharma - the Nirmanakaya - the Manifestation Body. But since they contain both suffering and end of suffering, ignorance and enlightenment, where then is the Enlightened mind of the Buddha. This is conceptualized as Sambhogakaya, the Reward Body, the mind of enlightenment as distinct from the totality.

Of course at the end everything is still all part of One Truth, the Dharma.

I think the previous answers are good but there's more to it.. The dharmakaya, sambhogakaya, and nirmanakaya, together called the trikaya are the key summary of Buddhism and in fact Taoism and the whole spiritual shebang. It encompasses all virtues, all powers, and all the things that you could possibly want.

One can work on each body and still not gain the others. They are interpenetrating but they are also worked on individually. This means that you can be Enlightened but not have any powers (effects of sambhogakaya and nirmanakaya).

I like to describe the Trikaya through the powers that one gains from perfecting them.

Body of Essence: Dharmakaya: being free from birth-and-death, no ego, disillusionment

Body of Virtue: Sambhogakaya: all the paramitas completed, all virtues completed

Body of Manifestation: Nirmanakaya: self-duplication (think Naruto), thought emanations, thought manifestation

The Zen masters all had the dharmakaya but usually did not complete the sambhogakaya or nirmanakaya. They were just like "Ah, I got everything. Worldly people and their affairs don't matter to me. Everything is emptiness. I am not real and neither are they. All that matters is my true self, which is indescribable. Off I go." But there was more to the Buddhist project than that!

Some Mahayanist masters on the other hand had the dharmakaya and the sambhogakaya, they exemplified stellar character yet also had deep realizations of the soul that the Zen masters had. They had achieved dharmakaya and sambhogakaya.

Lastly, some Taoist masters have the nirmanakaya but not the previous. They could perform miracles, thought manifestation, etc. Many could even live for thousands of years at a time. Nonetheless, they held too strongly to their notion of a self and did not see their true nature, their true self which exists beyond the body and mind. But they could live forever, so that's something isn't it? Wrong. The Buddha said that these people are living the wrong path and he could see deep into their future lives and said they would suffer repercussions in the future lifetimes for it. (Of course you and I don't any of these three so we'll suffer even worse repercussions I think! So don't be looking down on these immortals with this piece of knowledge you have just attained.) Many seers and people that can do things with their chi energies? They are all just playing with the nirmanakay body.

Ever mantra and feel like the Buddha is helping you? That's a nirmanakaya manifestation. A whole bunch of them.

Big secret: A Buddha actually completed the cultivation of Trikaya and stays in a separate dimension and sends out his nirmanakaya manifestation to people based on causes-and-conditions.

Big secret #2: We all actually have ALL three bodies, too. They are a range and you can see in your experience how much of each body you have achieved.

Each can be summarized respectively in the following qualities: true awareness, true virtue, true power.

As you climb the spiritual ladder, you will develop these qualities but with the Trikaya model and the other models in Buddhism, you can turn it into a much more fun project and work on what you're missing... in a very long-term sense since usually nobody accomplishes all three Trikaya in one lifetime.

For more on this subject of Buddhism with this kind of thinking read "How to Measure and Deepen Your Spiritual Realization" by Bill Bodri.

Definitely, know that the Trikaya are NOT just some psychological thing. They involve things like spiritual immortality, immortality of character, and immortality of the flesh-body (respectively). They umbrella the entire scope of development of everything with of course importance placed on the development of Buddhist emptiness for spiritual immortality and virtue and Taoism for cultivation of the power of multiple body emanation (yang shen).

I will say this is a very good explanation that may go beyond what your question is, but here is a very good summary from the Tantric tradition

The Three Kayas: The Bodies of the Buddha

THE FRUITION OF BUDDHIST PRACTICE is the realization of the three kayas--Dharmakaya, Sambhogakaya, and Nirmanakaya. These are the three bodies of Buddha's being or enlightenment. Dharmakaya corresponds with one's mind, Sambhogakaya with one's speech, and Nirmanakaya with one's body. Dharmakaya is the formless body. It is an undifferentiated state of being which we cannot talk about in terms of either confusion or enlightenment.

The Dharmakaya is something that is always present; it is rediscovered rather than created anew. Because it is atemporal and ahistorical, we cannot attribute change or transformation to it. Because it is passive and indeterminate in nature, Dharmakaya cannot manifest as a medium for one to work for the benefit of others, but it does give rise to the deterministic aspects of Sambhogakaya and Nirmanakaya.

The similarities between Dharmakaya and Sambhogakaya are represented here.

Like the Dharmakaya, the Sambhogakaya is always present. It has to do with mental powers, with the ability of one's mind to manifest in relation to the five wisdoms.

They are similar yet different

Dharmakaya and Sambhogakaya do not refer to entities so much as existential states of being, and only the Nirmanakaya body is created anew in physical form. Actually the three kayas are two bodies--the formless body and the body of form. Both the Sambhogakaya and Nirmanakaya are normally called the form bodies of the Buddha, while Dharmakaya is formless.

Here is closest to an answer to the question on differences.

The Nirmanakaya and the Sambhogakaya basically manifest out of Dharmakaya; in other words, both of the form aspects of the Buddha's being are dependent on the formless body. The Dharmakaya is the origin or field on which the other two are grounded.

and it takes this explanation further

When we call the Sambhogakaya a "form body," we do not mean physical form but instead form in the sense of manifesting and being determinate, as opposed to Dharmakaya, which is formless because it is not determinate. The Sambhogakaya is determinate because, although it is not physical, it does manifest in varieties of ways. If Sambhogakaya is fully realized, then one can receive different teachings and meanings from many natural sources, such as sound, sight, and so on.

Sambhogakaya in turn gives rise to Nirmanakaya, which is realized through the physical body, and embodies both the Sambhogakaya and the Dharmakaya aspects. Nirmanakaya is physical in its essence and is historically situated, so that when we talk about Buddha Shakyamuni attaining enlightenment in Bodh Gaya, giving teachings in Varanasi, and eventually attaining paranirvana in Kushinagar, we are describing his Nirmanakaya aspect.

Given by Ven. Traleg Rinpoche

This is a very lengthy article and worthy of further study.