Philosophical, mystical, theological and liturgical treatises and instructions. For example, the construction of the temple, the making of statues, the installation and consecration of the same, the course of rites and festivals, ways of salvation, cosmology are described.
There are three groups of agamas that define the respective doctrine – Vishnu Agamas worship Vishnu, Shiva Agamas Shiva, Shakti Agamas the Goddess, as the supreme cosmic power (Shakti).
Tantra, the philosophy about the Goddess, is part of the agamas. So one speaks of Tantra in the Shakti texts. In Shiva and Vishnu texts from Agamas.