illustration (attribution, if any possible, is at the end of the article)
Shingon means 'mantra' and is an esoteric school, from India via China, based on early tantra. It is resolutely among the Mahāyāna schools. It is noticeable that Tibetan buddhism developed the Vajrayana teachings independently. Many Indian 'sources' are shared between Shingon and Vajrayana, but there is no direct connection.
As in Tibetan tantra though, the relation between teacher and student is of extreme importance in Shingon, and teachings are 'secret' (not because they're hidden but because they should only be given in the situation of an appropriate teacher-student pair).
Meditative technics from tantra regularly rely on visualisation, and a central figure of Shingon is Vairocana or Mahāvairocana (in Japan, called Dainichi Nyorai 大日如來) as the central primordial buddha: a visualisation of thusness —impermanence, emptiness— and of the Dharmakāya —principle of Enlightenment).
In total, 37 deities are listed though.
Shingon also relies on mandalas. Two mandalas in particular are considered to summarise the whole of the Dharma, i.e. form the root/basis of Buddhism. These two mandalas naturally have sub-aspects ("9 mandalas" for the Vajradhatu, the "fourfold mandala" for the Garbhadhatu).
Shingon perceived itself as the 'complete' school of Buddhism while all other schools focus on particular aspects only (and thus are not necessarily wrong, but incomplete).
Shingon could thus accept the teachings of all other schools as partial components, which was useful when it appears in Japan after the "six schools" of the Nara period. The view of Shingon allowed to give sense to the collaboration between the six schools, while also claiming superiority over them for political reasons and patronage.
Tendai had also entered the scene during Shingon's development, and one of the teachings of Shingon —the ten stages of development of human consciousness— directly mapped different Nara schools as well as Tendai as stages 4 through 9, with Shingon being the 10th stage:
the 10 stages can be divided into 3 samsaric stages, 2 related to Hinayana, 2 related to Mahāyāna's "Three Vehicles", 2 related to Mahāyāna's "One Vehicle" and finally one associated to Vajrayana and Shingon.
Unsurprisingly, given the Indian tantric origin, Shingon also uses mantras and syllab-words (http://www.visiblemantra.org/vairocana.html).
As most tantric schools, Shingon relies on rituals: Shingon supports the notion that enlightenment is possible within this very life, based on the doctrine of buddha-nature, and rituals are thus keys to let defilements drop away and access the realisation of emptiness directly.
For all the claims of superiority and completeness, Shingon also acknowledged that tantra was a mere teaching device, maybe the complete one, maybe 'taught' by the Dharmakāya itself, but nonetheless just a teaching device.
The truth is directly tied to the Dharmakāya, not to any teaching which can be expressed! The truth is beyond words. It is the combination of the "teachings of eternal differentiation" and the "teachings of eternal sameness". It is one but dual, two but one. All dharmas are Mahāvairocana, the Absolute is identical to our own bodies, and Buddha-dharma is identical to our own forms.
(image via http://www.onmarkproductions.com/html/dainichi.shtml)