Modern scholars have attempted to cut this Gordian knot, but the lines are blurred and it is sometimes impossible to tell where one school or tradition ends, and another begins.

One of the best attempts to classify the broad group of texts currently available was made by Jan Gonda in the History of Indian Literature (Wiesbaden: Harrassowitz 1977). The chapters of this work look at the various texts under the heading of Agamas, Vishnuism, Bhakti movements, Pancharatra, Vaikhanasa, Shivaism, Shaivite Agamas, individual Agamas, works of yogic schools such as the Nathas; Pashupatas; Dattatreya bhaktas, Stotra literature and miscellaneous works including gitas (songs) and other works of the like. This is a valiant and heroic attempt to look at a large number of texts still in existence.

Hindu Tantrism, compiled and edited by Teun Goudriaan, Dirk Jan Hoens and Sanjukta Gupta, specifically looks at Hindu tantrik literature, classifying it into tantras of the Shri Kula, the Kali Kula, and also dealing with "magical" tantras, mantra shastra Vaishnava tantras and tantras ascribed to other sects. This work looks at contemporary tantrik literature in modern Indian languages, such as Hindi and Bengali. (Brill, Leyden 1979).

Others (Agehananda Bharati in his The Tantric Tradition, Rider & Co, 1965), make the picture a little cloudier by insisting that sexual contact is the main point of tantrism, whether Buddhist or Hindu.

"The central sadhana of tantrism, Buddhist and Hindu alike, is the exercise of sexual contact under tantric 'laboratory' conditions. It is irrelevant, in the final analysis, whether these sadhanas were or are literally performed, or whether they are hypostasized entirely into mental configurations." (Bharati, Chapter IX).

Georg Feuerstein, in his far more recent book Tantra: The Path of Ecstasy (Shambhala, 1998), takes a broader view: "At one end of the Tantric spectrum we have highly unorthodox practices such as black magic that go against the moral grain of Hindu society (and that of most societies). At the other end we have Tantric masters who decry all doctrines and all rituals and instead applaud the ideal of perfect spontaneity (sahaja). Most schools fall between these two poles; they are typically highly ritualistic but infused with the recognition that liberation springs from wisdom, which is innate and therefore cannot be produced by any external means." (Op. cit, Introduction, page 10).

Many modern commentators define tantrik practice -- Hindu tantrik practice that is -- in terms of the so-called Right Hand (dakshinachara) and Left Hand (vamachara) rites they perform. Under this attempt at classification, the latter belong to schools in which rites such as the panchatattva have a place; sexuality being a core part of these rituals. So-called Right Handed Schools are said to use either ritual substitutes or to eschew these practices completely.

Even this isn't as straightforward as it seems. For example, there is a major school of the Shri Vidya tradition, the Samayachara, which goes out of its way to roundly condemn any such practices, will not even contemplate meditating on chakras below the navel such as the Muladhara and Svadisthana, and even worship the yantra with the apex pointing upwards to avoid giving the impression that the symbol or the goddess may have anything to do with sexuality.

In the introduction to the well known Saundarya Lahari, (The Ocean of Beauty) published by the Theosophical Publishing House Adyar, in 1972, we find the following comments:--

"The votaries of the Shakti, the Kundalini, may be roughly divided into two classes: the Samayin-s or those who believe in the sameness of the Shakti and the Shiva, and the Kaula-s or those who worship the Kaulini, the sleeping Kundalini, i.e. the Shakti, which resides in the Muladhara, which is known as the Kula-plexus...The Kaula-s.. worship the Kundalini, even without rousing her from sleep and are satisfied with the attainment and enjoyment of purely temporal objects, believing, at the same time, that with the rousing of the Kundalini, they attain liberation." (Introduction, page 3, Pt S. Subrahmanya Sastri).

This might come as a surprise not only to the Kaulas (clans) often associated with so-called "left hand worship" but to those adherents of another highly important school of "tantra", the Kaulas of the Trika School of Kashmir, never mind to those texts often ascribed to the Kaula schools, such as the Kularnava Tantra, which certainly don't hold the Samayin view above, or to followers of the schools of Matsyendranathapada.

Further, the vast body of literature describing or related to Shri Vidya is not the homogeneous whole you might expect. There are different divisions and subdivisions within schools and traditions worshipping Lalita which may be based on differences in the mantra or vidya, in the approach to the symbol of the Shri Yantra, or to differences in the worship. The Gandharva Tantra, for instance, advocates the worship of a Kula Shakti, while Bhaskaraya, who wrote an extensive commentary on the Thousand Names of Lalita, describes himself as a Kaula.

Sir John Woodroffe, who under the pen name Arthur Avalon translated several tantrik texts, and wrote other books on the subject in the early 20th century, tends to another classification, to which he constantly returns -- that of divyas (divine), viras (heroes) and pashus (the herd). It is only in virasadhana, or work relating to sadhana in the heroic mode, that lata sadhana, that is to say sexuality, has a place.

Yet even this is not a rule. Indeed, some tantras, such as the Brihad Nila Tantra, advocate a type of worship called Mahachinachara, which appears to have no rules whatever.

The traditions in different parts of the large sub-continent may vary widely. Woodroffe seems to have learnt from a number of Bengali tantriks and pandits, and his books are coloured by these views. The 10 Mahavidyas popular in Bengal, and other parts of India, for example, do not form any kind of a rule. In the Kashmir tradition, while Kalika may be hailed as the Adya Shakti, her forms in works such as the Tantraloka of Abhinavagupta and the Chidgaganachandrika, entirely differ in name and image from Dakshina Kalika, the Kamakala Kalika, and other of her forms encountered in many Bengali texts.

As we mention in the introduction to these Web pages, as if this wasn't confusing enough, the modern use of tantra in India (to describe black magic) and in the USA (to describe 'sacred sexuality') offer further traps for the unwary.

The texts themselves, as we mention earlier in this piece, do not clarify the position. Some take a blatantly sectarian stance, while others (such as the Mahakalasamhita) mention many divisions and schools that can hardly be traced.

Cults have grown up around local deities, or around the Big Five Indian deities: Shiva, Shakti, Ganapati, Vishnu and Surya (the Sun). In this latter division, there are many sub-sects, schools and divisions, "innumerable", as Devi says in the Kulachudamani Tantra. Many of the texts describe themselves as dialogues between Shiva and Shakti, yet other sects, schools and traditions may say that this is pure delusion.

Some works which describe themselves as tantras are, quite simply, collections of spells. One, calling itself the Dattatreya Tantra after the famous guru figure of India, is roughly comparable to mediaeval Western grimoires. You won't find much or anything in there about yoga, spirituality or any type of inner search. The Hindu tantrik divisions, nevertheless, do have one thing in common and that is the importance of the guru - one with the Devi, the Deva and ultimately the disciple or shishya. This, of course, has led to abuses and misunderstandings of its own. While many of the tantrik texts prescribe the qualifications of both guru and disciple, the truth is that an aspirant is really on her or his own.

There simply aren't any guarantees, nor, as we have sketched out above, is there any agreed set of rules or catechism which give a handy pat meaning to the word tantra.
http://www.clas.ufl.edu/users/gthursby/tantra/schools.htm