Now, today's celebration really causes confusion to non-Catholics. Basically, we believe that Mary spent her life from the moment of conception to her Assumption without the taint of Original Sin. She is the only human, aside from Jesus, Adam, and Eve to be created without it.
Why does she have that special privilege? As told in today's homily, God was able to choose His mother. Think of the consideration He would take in the creation of His own mother. He formed her before she was ever born. Meanwhile (as I have learned in the class on Apologetics), Mary is the New Testament's Ark of the Covenant. God gave Moses specific instructions on how to build and handle the Ark, and it was made only to carry the Ten Commandments (and to give God a place to sit from time to time). Mary was created to carry God Himself. How much more care would God put into the creation of such a vessel?
Mary is certainly a special person.