One of the important traits of Indian culture is its inclusivity. It cannot be otherwise in a place where everything from grass to galaxy is viewed as the two manifestations of the same supreme power.
One of the misunderstood sects inside Hinduism is Saktheya or Tantric cult. They are considered in worst manner even now. I do not deny the fact that many rituals in Tantric way have lost their inner sense in the hands of few. But the fact is that, it has happened to almost all religious sects across the world.
The point here is the philosophy of sakthi worship. God is considered as the universal mother and worshipped in Sakteya way. Mother worship is one of the oldest in any civilization. But here, its philosophy has improved over times and is just near Advaita(Non-Dualism).
In Sankaracharya's advaitha, which is referred as 'Kevala Advaita', world is a mere fantasy. It appears, but it is non-real. The example used to substantiate this is 'Rajju Sarpa Nyaya'. In comparitively darker place, a person seeing a rope is considered here. He thinks it as a snake. He feels it is moving, and all the emotions a person could have on seeing a snake happens to him. On the arrival of light alone, will he be relieved from this imagination.
On similar grounds, the world is nothing but pure 'Brahman' and nothing else. But due to lack of knowledge, we impose various attributes to it. All rituals,bhakthi(devotion),Karma(Action), and everything else that scriptures preach is for the attainment of this knowledge.
The Sakteya cult also is believers of non-duality. But it has a slight difference. They consider all as the manifestation of one supreme power, but they do not negate the world(As is done by Advaitins-'Neti Neti' -Not this, not this principle). Instead it considers whole world is just the manifestation of lord in various forms. Hence it attains a harmony between this world and the real essence of it.
Sakteya cult is mainly split into two-Samayins and Koulas. Samaya cult considers Siva and Sakti as equal(Sama). Siva is considered as the Supreme being and Sakti is the Supreme Will. According to Samayins, both are equal. Their worship is expected to be in 'Daharakasha'-Space inside the heart. That means they are expected to do no rituals outwardly. But most of them do it now. Koula on the other hand consider the supremacy of Sakti over Siva. This is evident from the Kali's portrait in which she stands over Siva. For them the external rituals are must to invoke the Kundalini. They are the people who are misunderstood widely, for their rituals involving Liqour,Meat and sometimes even Sex. The lack of knowledge on those areas were making in pre-mature judgements.
The Sakta way is three-fold. They are Mantra,Yantra and Tantra. Mantra is the sounds orderly arranged so as to invoke the powers.Yantra is the Pictorial represntation of the energy patterns of the sounds. Tantra involves other rituals including Pooja.
The sakta cult is involved with invoking Kundalini sakti. They believe that there is an immense power source inside human body which is coiled up in the form of a Serpent at the lowest part of spinal chord-near the anal part which is called 'Mooladhara Chakra'. The body has three major power channels-Ida,Pingala and Sushumna. Sushumna passes through spinal chord. Ida and pingala are proven to be physically present.
The power in human usually travels through Ida and Pingala. If we can block them, through Pranayama and force it to enter through Sushumna, which normally would be closed at the end, then the Kundalini will rise.
The kundalini then travels up through 5 other Chakras to ultimately reach Sahasrara Padma-thousand petaled Lotus, where it will be united with Siva and then that man attains Liberation.
This is the basic principle on which Sakta cult is built on.
Soundarya Lahari is a work by Sankara Himself, which deals with this cult and its Philosophy in a highly poetic language.
Sir John Woodrof(arthur Avalon)'s works are also Authentic in nature.