What is the main goal for Theravada Buddhism and Zen Buddhism? Is it Nirvana for both or is there any additional differences?
Can the answer be detailed because this is an assignment and I am trying to be as detailed as possible. Thanks so much!
There is no goal in Zen. To have a goal is spin the wheel of samsara. Zen is all about getting to the basics, which are beyond words and concepts. I should know I have been practicing Zen for seventeen years and I am still a beginner.
And what is the noble search? There is the case where a person, himself being subject to birth, seeing the drawbacks of birth, seeks the unborn, unexcelled rest from the yoke: Unbinding. Himself being subject to aging... illness... death... sorrow... defilement, seeing the drawbacks of aging... illness... death... sorrow... defilement, seeks the aging-less, illness-less, deathless, sorrow-less, undefiled, unexcelled rest from the yoke: Unbinding. This is the noble search. -Ariyapariyesana Sutta
"aging-less, illness-less, deathless, sorrow-less, undefiled, unexcelled rest from the yoke: Unbinding" here refers to Nibbana. So yes, it is the goal in Theravada Budhhism.
I think this question might go unanswered so I'll give it a try.
This answer isn't detailed, but it's how I might structure an answer. The structure is kind of historical. For more details you can google for words I quoted (for example sila).
The earliest Buddhist Dhamma is maybe the "Four Noble Truths": which are about suffering or unsatisfactoriness ("dukkha"), the cause, and the cessation of suffering -- and the "Noble Eight Way", which is the path leading to the end suffering.
So the goal is to end suffering ... or to complete the way, to do what needed to be done etc.
The "Noble Eightfold Way" can be summarised as the "Threefold Training". Some of the training is seen as being more intended for monks than for laypeople.
The bit intended for everyone (including laypeople) is morality also known as "sila" (including "the precepts") -- of which you get summaries like this or this. These are maybe not the goal (or maybe they are the goal for laypeople), but a goal. Or maybe the goal is "sukha", maybe a good rebirth.
The other two parts of the three-fold training are concentration and wisdom: so those might be called goals too.
The "suttas" (which are transcripts of the Buddha's doctrine) are fairly long. Two other parts of the "Tripiṭaka" are the "Vinaya" (the monastic rules, for monks and nuns, also from the Buddha) and the "Abhidhamma" (which I think is a scholastic analysis or reworking, commentary, on material in the suttas). It's quite lengthy.
I'd say that it (at least the suttas) is almost all about the Way, rather than about the Goal.
The Way includes abandoning "fetters", also "hindrances" and "outflows" and so on. Because these are the Way, these too could be seen as goals.
Nibbana is one of names for the Goal -- it's one of many names.
I think that, later, people discussed the metaphysical nature of nibbana: "does it exist?" etc.
Buddhist doctrine migrated via China. Some scholars say it was influenced there by Taoism. From there it reached Japan. So fas as I know, at some point some Japanese masters decided there was too much scholarship (too much text, perhaps) ... and put more emphasis on meditation. It continued the tradition of learning from teachers ("lineage"), living in monasteries, and so on.
One of the early (Theravada) doctrines (from the Four Noble Truths) is that suffering is caused by desire, that suffering ceases when desire ceases: so you get suttas like this. I don't know much about Zen, I think that one of its doctrines is to meditate without a goal (which may be similar to "without desire", and is relevant to your question). Another value seems to be spontaneity -- maybe not a goal in itself but a characteristic of an "enlightened" state of mind -- which ("spontaneity") is, maybe, not one of the explicit Theravada values? I'm not sure whether to see this as a lack of "attachment" (as might be understood by Theravada), and/or as something with a Taoist influence.
Zen Buddhism doesn't only exist in Japan: also in Vietnam and Korea. A reason that Zen is famous in the English-speaking world is that it was one of the first forms of Buddhism in the United States.
All that said, the goal might be similar. Here's a quote from Thich Nhat Hanh (a famous Vietnamese Zen monk):
Oprah: Thank you for the honor of talking to you. Just being in your presence, I feel less stressed than when the day started. You have such a peaceful aura. Are you always this content?
Nhat Hanh: This is my training, this is my practice. And I try to live every moment like that, to keep the peace in myself.
Oprah: Because you can't give it to others if you don't have it in yourself.
Nhat Hanh: Right.
Oprah: I see. I know that you were born in Vietnam in 1926. Is there any wonderful memory of your childhood that you can share?
Nhat Hanh: The day I saw a picture of the Buddha in a magazine.
Oprah: How old were you?
Nhat Hanh: I was 7, 8. He was sitting on the grass, very peaceful, smiling. I was impressed. Around me, people were not like that, so I had the desire to be like him.