How is Japanese Shingon Buddhism different from other forms of Buddhism such as Zen?
Shingon is a tantric Buddhism, which makes it rather dissimilar to Zen and a close relative of Tibetan Vajrayana. Unlike Theravada and Mahayana, both of which rely on discipline, study, and meditation for achieving Enlightenment, tantric systems work by putting a student inside a carefully crafted mandala, and having him or her interact with its energies.
Before the student can be introduced to his mandala, he needs to establish a deep relationship with the teacher. Once teacher has learned student's natural inclinations and energy profile, he picks a suitable tantra, tweaks it to match the student, and introduces the student to various deities and energies. As mandala begins to emerge, student works with it by participating in empowerment ceremonies, performing various rituals, and doing visualizations.
If teacher sees student making good progress, he will upgrade him/her to a higher tantra, and the whole process repeats until the student attains complete vision of Truth.