In the following discussion we are specifically talking about the shakta and shaivite sects, with the emphasis on the shaktas.
Shiva represents the unmanifest and Shakti the manifest. Shiva is the formless and Shakti is the formed. Shiva is consciousness and Shakti is energy, not only in the cosmos as a whole, but in each and every individual. Shiva is the supreme consciousness pervading each individual and Shakti is the power that manifests the potential contained within the consciousness in different manifested centres within man and the universe. The roots of Shakti are in Shiva. Though one is manifested and the other unmanifested, they are in the ultimate sense one and the same. One is the principle of changelessness and the other is the principle of change. Shakti is change within changelessness and Shiva is changelessness as the root of change. This relationship seems contradictory in logical terms, yet the experience of perfect oneness and unity of the changeless and the changeable, the dissolution of duality, is the aim of tantra as well as yoga. There is ultimately no difference between Shiva and Shakti, but this is not realizable except in states of higher awareness. And this realization transforms life into a source of wonder and joy.
Everything that you see around you, everything that is physical, psychic, mental or whatever, is Shakti both individually and collectively. This includes your mind and body. It includes every tree, every dog, every stone, the sun. Shakti is everything. All these mani-festations have come from the underlying substratum, Shiva. The aim of tantra is to retread, in a sense, the path of manifestation, so that Shakti is the vehicle to reunite the individual with Shiva and know the supreme experience, samadhi, nirvana or whatever you want to call it. Tantra aims to lead you on the path back to union of Shiva and Shakti, union of the individual being (jiva) with the paramatman (supreme).
Tantra says that Shakti, or the power of creating separate centres of manifestation (i.e. objects, individuals, etc.) is in essence consciousness itself (Shiva). However, the power of the phenomenal world around us veils itself (through maya), and it is not obvious at ordinary states of awareness that it is indeed consciousness. Each and everything in the created universe is no more than manifested consciousness. But strange though it seems, there is no change in the nature or content of consciousness, even though everything comes from it. In the Ishavasya Upanishad it says:
That is full, this is full.
From full, the full is taken,
the full has come.
If you take out the full from the full,
the full alone remains.
The great ancient sage of Europe, Plotinus, said exactly the same thing: “The one is every thing and also not every thing. It is not every thing because it is the source of every thing. It is transcendentally every thing, because there every thing is … , or, more exactly, is not yet but is to be.”
These two quotations are the essence of tantra and every religion. Don’t worry if you don’t understand their meanings, for what they are trying to convey defies intellectual speculation.