The word means "ancient", and the texts themselves define what a Purana should contain: the pancha lakshana or five marks. Sarga, the primary creation of the world; pratisarga, its re-creation after each dissolution; vamsha, the genealogies of gods and sages; manvantara, the vast cycles each presided over by a Manu; and vamshanucharita, the histories of royal dynasties, given for the dynasties of the sun and moon and continued, remarkably, in the future tense for kings who ruled after the texts claim to have been composed.
The definition fits the actual texts poorly, and the mismatch is informative. In most of the eighteen, the five marks occupy a small fraction of the material. The bulk is something else: instructions for vratas, vows and observances tied to the calendar; tirtha literature praising particular pilgrimage places and specifying the merit of bathing there; temple ritual, image installation, gifts, funerary rites, genealogies of sects, and long sectarian narratives establishing one deity as supreme. Some contain grammar, medicine, iconometry and architecture. The Agni Purana is close to an encyclopaedia.
The eighteen are conventionally listed as Brahma, Padma, Vishnu, Shiva, Bhagavata, Narada, Markandeya, Agni, Bhavishya, Brahmavaivarta, Linga, Varaha, Skanda, Vamana, Kurma, Matsya, Garuda and Brahmanda, with variation in the lists themselves, since some substitute Vayu for Shiva. Sizes vary enormously; the Skanda Purana, at some 81,000 verses, is the largest of them and effectively a library of local temple traditions. Below the Mahapuranas sit dozens of Upapuranas, including the Devi Bhagavata, and below those the sthala puranas of individual temples, which are still being composed.