In Roman Catholic teaching, "original sin" is the sin of Adam which humans inherit at conception but which is washed away in Baptism. "The Immaculate Conception" is the doctrine that Mary was preserved from original sin at the moment of conception, receiving in advance the grace of Baptism that her son would earn for all mankind.
Images that celebrate this doctrine represent Mary as the woman of Revelation 12:1, "And a great sign appeared in heaven: A woman clothed with the sun, and the moon under her feet, and on her head a crown of twelve stars."
The sculpture at left is typical. It shows the crown of twelve stars and represents the sun as a modified mandorla outline with flame-like "rays." The creature at the base is the dragon who assails the woman in Revelation 12 (cached). Two-dimensional images represent the "heaven" in which the woman is seen by surrounding her with angels and often by including clouds and a sky-like background (example).
Often the Virgin stands on a globe or on a cluster of clouds or angels rather than the moon, possibly because the moon is associated with the Virgin of the Assumption. The latter type is also derived from Revelation 12 but does not as a rule include a dragon or crown of stars. A mandorla may be used in either type.
Sometimes the image is called "Maria Tota Pulchra," the opening words of one of the antiphons for the psalms of Vespers on the Feast of the Immaculate Conception: Tota pulchra es, Maria. Et macula originalis non est in te ("You are all beautiful, Mary, and there is no original blemish in you"). The antiphon comes from a passage attributed to John Damascene in the Golden Legend.