Answer:
Many people confuse the Immaculate Conception with the virgin conception of Jesus. However, these are two very different things.
According to Roman Catholic theology, the Immaculate Conception is, the conception of Mary without any stain (macula in Latin) of original sin. That is, Mary was conceived and born without Original Sin.
Theology states that, from the first moment of her existence, she was preserved by God from the lack of sanctifying grace that afflicts mankind, and that she was instead filled with divine grace. It is further said by Catholics that she lived a life completely free from sin.
Her immaculate conception in the womb of her mother, through sexual intercourse, should not be confused with the doctrine of the virginal conception of her son Jesus, known as the Annunciation, and followed by the Virgin Birth.
An official statement of the doctrine reads, "…the blessed Virgin Mary to have been, from the first instant of her conception, by a singular grace and privilege of Almighty God, in view of the merits of Christ Jesus the Savior of Mankind, preserved free from all stain of original sin." Essentially the immaculate conception is the belief that Mary was protected from original sin, that Mary did not have a sinful nature, and was, in fact, sinless.
So how did it happen? God wanted Mary to be the mother of Jesus and he wanted to make sure that she was an appropriate vessel. He therefore gave her divine grace as she was conceived and made sure there was no stain of Original Sin.