Tantra has been mainly a folk religion. The Tantrikas were believers in Yoga and the opening of cakras, and worshipped Siva-Sakti, or, more usually, Devi, the mother-goddess.
The collection of Tantrika lore is called Samhita or the Agamas, which means that it has come from before (through a long line of teachers and disciples). The Agamas has four angas (limbs) or portions.
The first limb is knowledge. Here it is closest to the Upanishads. The Brahma doctrine that we find in the Mahanirvana Tantra is a mixture of the philosophies of the Upanishads and the Samkhya. The second limb is Yoga. Kriya, or practices, form the third limb. The fourt is Carya, or conduct or character, which deals with man’s social and personal conduct.
Women play an important part in the Tantras, which, unlike many branches of orthodox Hinduism, do not allot women an inferior status in religious matters.
Source: Hinduism by K.M. Sen (1976, first edition 1961).