A Mother Goddess is a goddess portrayed as the Earth Mother who serves as a general fertility deity, the bountiful embodiment of the earth.
From the elegant snake-offering goddess figures of Knossos to the rock-cut images of Cybele, to Dione ("the Goddess") who was invoked at Dodona, along with Zeus, until late Classical times, it is sometimes too facile to class all archaic female goddesses as manifestations of the Mother Goddess. Archaeologists tend to avoid such theories in interpreting sites and material remains, and sometimes denigrate them as inspired more by radical feminism than strict archaeology.
Matriarchy and Goddess history
Mother Goddesses have been revered in nearly all societies, though James Frazer (author of The Golden Bough) and those he influenced (like Robert Graves and Marija Gimbutas) advanced the theory that all European and Aegean Mother Goddess worship had originated in neolithic matriarchies. This has been discounted by modern scholars, most notably by Peter Ucko  (http://www.debunker.com/texts/goddess.html). The actual cultural and religious context of neolithic figures like the Venus of Willendorf has not been established. Some authors believe they were intended to represent goddesses, while others believe that they could have served some other purpose. The extreme position in the modern de-mystifying of these objects is the suggestion which has been made that they were children's toys.
Whether those figures were venerated as mother goddesses or not, there is no dispute that many ancient cultures worshipped female deities as part of their pantheons: Ishtar (Inanna), in Mesopotamia, (Asherah in Canaan, ‘Ashtart in Syria, Aphrodite in Greece), is well-known. In Scandinavia a female goddess was worshipped during the Bronze Age and later in Norse mythology as Freya. Other female goddesses in different pantheons may also be considered mother goddesses.
In the Aegean, Anatolian, and ancient Near Eastern culture zones, a Mother Goddess was worshipped in the forms of Cybele (revered in Rome as Magna Mater, the 'Great Mother'), of Gaia, and of Rhea.
The Olympian goddesses of classical Greece had many characters with Mother Goddess attributes, including Hera, Demeter and Athena. In Minoan Crete one of her aspects was the Mistress of the Animals (Potnia Theron) who devolved into the huntress Artemis; the archaic Artemis of many breasts worshiped at Ephesus retained some of her older aspects.
In the Hindu context, the worship of the Mother entity can certainly be traced back to early Vedic culture, and perhaps even before. Today, Devi is seen in manifold forms, all representing the creative force in the world, as Maya and prakriti, the force that galvanizes the Divine Ground of existence into self-projection as the cosmos. She is not merely the Earth, though even this perspective is covered by Parvati. All the various Hindu female entities are seen as forming many faces of the same female Divinity.
This form of Hinduism, known as Shaktism, is strongly associated with Vedanta, Samkhya and Tantra Hindu philosophies and is ultimately monist, though there is a rich tradition of Bhakti yoga associated with it. The feminine energy (Shakti) is considered to be the motive force behind all action and existence in the phenomenal cosmos in Hinduism. The cosmos itself is Brahman, the concept of the unchanging, infinite, immanent and transcendent reality that is the Divine Ground of all being, the "world soul". Masculine potentiality is actualized by feminine dynamism, embodied in multitudinous goddesses who are ultimately reconciled in one.
The keystone text is the Devi Mahatmya which combines earlier Vedic theologies, emergent Upanishadic philosophies and developing tantric cultures in a laudatory exegesis of Shakti religion. Demons of ego, ignorance and desire bind the soul in maya (illusion) (also alternately ethereal or embodied) and it is Mother Maya, shakti, herself, who can free the bonded individual. The immanent Mother, Devi, is for this reason focused on with intensity, love, and self-dissolving concentration in an effort to focus the shakta (as a Shakti worshipper is sometimes known) on the true reality underlying time, space and causation, thus freeing one from karmic cyclism.
Mother Goddess worship in Christianity
Some people consider Mary to be a "mother goddess", being that she fulfills not only a maternal role but is often viewed as a protective force and divine intercessory for humanity.
Some Latter Day Saint denominations give reverence to, and (though rarely) worship, a Heavenly Mother.
The Mother Goddess, amalgamated and combined with various feminine figures from world cultures of both the past and present, is worshipped by modern Wiccans and other Neo-Pagans. The mother goddess is usually viewed as mother earth by these groups.