Sometime between the year of 4 and 6 BC, a Galilean Jewish woman, Mary, gave birth to our Lord Jesus Christ.
Mary has always been referred to as the Theotokos, literally meaning the God bearer, and for very good reason. She did not simply give birth to a human being, but through the incarnation, she gave birth to our Lord who is both fully God and fully man. She gave birth to the Word made flesh. Therefore it is correct to describe Mary as the Mother of God since, theologically (and technically) speaking, that is precisely who she is.
Mary is not God. She is not our Saviour. Yet she has an important role in our salvation.
Many Christians, especially protestants, try to cut the knot out of the difficulty by referring to Mary as simply the "earthly mother of Jesus" - as if her life had no real significance. She was certainly Jesus's "earthy mother", but scripture tells us some other important things about Mary.
Biblical scholars will be familiar with "types", things written in the Old Testament which foreshadow that in the New. Many texts in the NT have interesting and interwoven parallels with those in the OT. Word for word, the events surrounding the account of Elizabeth and Mary in the first chapter of Luke closely mirror those of David and the Ark of the Covenant.
For me, Mary is much more than simply the "earthly mother of Jesus". Especially chosen by God, she was the virgin who carried the Lord Jesus Christ, God incarnate, in her womb. Mary is the Ark of the New Covenant. She is not to be worshipped, since our worship should be directed to God alone, but she is to be venerated and respected.