It’s important to understand what the doctrine of the Immaculate Conception is and what it is not. Some people think the term refers to Christ’s conception in Mary’s womb without the intervention of a human father; but that is the Virgin Birth. Others think the Immaculate Conception means Mary was conceived "by the power of the Holy Spirit," in the way Jesus was, but that, too, is incorrect. The Immaculate Conception means that Mary, whose conception was brought about the normal way, was conceived without original sin or its stain—that’s what "immaculate" means: without stain.
It's interesting to note that the Church places greater importance on this day, when she commemorates the conception of Mary in the womb of St. Anne, than on the day she commemorates Mary's birth — celebrated, appropriately enough, nine months from today, on September 8.
In honor of the celebration of Our Lady's Immaculate Conception, I've included below what is, in my opinion, the most cogent paragraph ever written on Marian devotion.
It's from The Everlasting Man, my favorite work of G. K. Chesterton.
Given that we're in the midst of Advent, it's also rather timely:
If the world wanted what is called a non-controversial aspect of Christianity, it would probably select Christmas. Yet it is obviously bound up with what is supposed to be a controversial aspect (I could never at any stage of my opinions imagine why); the respect paid to the Blessed Virgin. When I was a boy a more Puritan generation objected to a statue upon my parish church representing the Virgin and Child. After much controversy, they compromised by taking away the Child. One would think that this was even more corrupted with Mariolatry, unless the mother was counted less dangerous when deprived of a sort of weapon. But the practical difficulty is also a parable. You cannot chip away the statue of a mother from all round that of a newborn child. You cannot suspend the new-born child in mid-air; indeed you cannot really have a statue of a newborn child at all. Similarly, you cannot suspend the idea of a newborn child in the void or think of him without thinking of his mother. You cannot visit the child without visiting the mother; you cannot in common human life approach the child except through the mother. If we are to think of Christ in this aspect at all, the other idea follows as it is followed in history. We must either leave Christ out of Christmas, or Christmas out of Christ, or we must admit, if only as we admit it in an old picture, that those holy heads are too near together for the haloes not to mingle and cross.