One of the most common misconceptions about the Catholic Church is that we worship Mary. Simply put, Mary is a saint and we intercede through her, we do not worship her. There is a long tradition tied to Mary, we believe so much about Mary, from her Immaculate Conception and her Assumption.
Starting from the beginning, Mary is the daughter of Sts. Anne and Joachim. There is no historical evidence of them, it comes primarily from legend and tradition of the early Church. The original story is taken from the Gospel of James, an apocryphal document, that says an angel appeared to tell them they would have a child and that Anne dedicated the child to God, similar to Samuel's dedication to the temple by Hannah (1 Kings).
When Mary was conceived, she was conceived without sin. This doesn't mean that she wasn't conceived like any normal human, but that her spiritual conception Mary was preserved exempt from all stain of original sin. She was blessed with sanctifying grace before sin could take effect in her soul.
Breaking it down, spiritual conception is when Mary's soul was created and infused into her body. Original Sin is Adam and Eve's sin, the first sin in the world that each human is born with. Catholics baptize babies to cleanse them of Original Sin. Mary did not need a baptism because she already received the grace that is given by baptism from God because God needed a perfect vessel to carry God's Son, a woman who was in a state of constant grace. Therefore, it isn't that Mary's Original Sin was taken away, it is that it was never there.
Mary, conceived without sin, pray for us.