Extracted from the website of Buddhist Vajrayana Charity Funds Association and translated (密宗細解) into English:

An Elaboration of Tantric Buddhism

According to the great Buddhist teacher Je Tsongkhapa, there are ten explanations for ‘Tantric Buddhism’:
1. Honourable Tantra
  Each Buddha or Bodhisattva of Tantric Buddhism has His own Mandala. It is like an imperial palace of a king which does not allow people to visit or tour around. The mantra and mudra of every Buddha or Bodhisattva is like an imperial seal of which not ordinary people can get hold. Hence, it appears to be in secret.
2. Profound/Subtle/Mysterious Tantra
  The Dharmadhatu as the environmental cause of all phenomena, the relationship of the mutual utilities among things is all very minute. In Tantric Buddhism, different ways of learning for cultivation are revealed in detail to enable human to know and understand it gradually. Ordinary persons find it difficult to understand because of their carelessness as well as mindlessness in learning. Therefore, Tantric Buddhism is thought to be implicit.
3. General Tantra
  The principles, the substance and the reality of Bhutatathata (Tathata /Suchness) are so widespread at all times that common people employ them daily without realizing it. On the other hand, guidance is needed in every aspect of Tantric Buddhism which makes it sounds like a secret.
4. Implicit & Explicit Tantra
  Inside a Dharma, it is actually endowed with ten-thousand Dharmas and among the principles involved, some are hidden whereas some are made explicit. In this way, sentient beings will easily cling to one thing but lose sight of another because of their incapacity to observe the subtlety. They tend to attach to one side (i.e. either Exoteric or Esoteric Sect) instead of seeing the Exoteric as the explicitness among the implicitness and the Esoteric as the implicitness among the explicitness. Since ordinary people are unable to realize this, they think that it is implicit.
5. Dharani Tantra
  One Dharma can gather and keep all meritorious virtue while innumerable Dharmas can be collected into one Dharma. When both efforts are exerted together, a complete fruition will be possessed as expected. However, only those who are good at learning manage to realize this. Thus, it is called a secret.
6. Heart-giving Tantra
  Having understood thoroughly the cause and effect of heart-giving in attaining Buddhahood, if one who is able to comprehend the meaning of teachings as well as the methods of practising stages of cultivation determines to practise cultivation, one will often obtain great results within a very short time. Nevertheless, common people may not know the function of this kind of initiating the bodhi mind (Bodhicittopāda 發菩提心). Perhaps, they have realized it but make no intention in doing it or they may have carried it out in a way that is not according to the doctrine. That is why it is named a mystery.
7. Stage Tantra
  Stages are exactly skillful means. As one can handle matters sophisticatedly and also cultivate the various practices which one manages to achieve correspondences in them all, one’s obstacles are eliminated immediately and the stages of enlightenment are instantly attained. The third-raters have no idea of it and so it is regarded as a secret.
8. Ignorant Tantra
  Without understanding the meaning of Tantric Buddhism, some people think that it is like witchcraft. They have never attempted to understand it personally before they make the improper comments. They look at Exoteric Buddhism and Esotericism with different views. Having inferior wisdom and deluded consciousness, they are attached to the former while defaming the latter. Consequently, Tantric Buddhism is seen as a mystery.
9. Covered Tantra
  Similar to fine wine which will have a better taste if it is sealed more firmly, the same applies to Tantric Buddhism. When one fights tenaciously to keep the Percepts, one will experience the perception at a faster rate and will attain greater achievements. Thus, it is suitable to be kept closed (like keeping a secret).
10. Prudent Tantra
  If Tantric Buddhism is disclosed absurdly, one suffers from a lot of afflictions and anxieties. The various immature conditions of sentient beings can be separated into deficiency in accumulation, greenness in the completion of sadhana as well as immaturity of link from exertion (Sanskrit word: vyavasaya). In particular; it is the lack of confidence in Tantric Buddhism which is due to the immaturity of cause. Hence, all Dharmas that should be kept in secret ought to be hidden. Otherwise, you may even lay yourself open to ridicule slandering the true Dharma (Sanskrit word: saddharma-pratiksepa). In that case, you mislead not only yourself but also other people. For the sake of being benevolence, one has no choice but to keep the secrets.
Doctrines that are taught publicly are Exotericism () whereas the cultivation practices of oneself are regarded as  Esotericism ().”