Catholics believe in the Immaculate Conception. What is that?
Is this the same as the virginal conception of Jesus?
The Immaculate Conception is not the same as the miraculous conception of Jesus by the power of the Holy Spirit.
The Immaculate Conception refers to the conception of Mary: Mary was conceived (by her mother, Saint Anne) being full of sanctifying grace and free of any stain of original sin.
It does not refer to any of the following, which Catholics also uphold:
From the Baltimore Catechism:
Q. 268. Was any one ever preserved from original sin?
A. The Blessed Virgin Mary, through the merits of her Divine Son, was preserved free from the guilt of original sin, and this privilege is called her Immaculate Conception.
Q. 271. What does the "Immaculate Conception" mean?
A. The Immaculate Conception means the Blessed Virgin's own exclusive privilege of coming into existence, through the merits of Jesus Christ, without the stain of original sin. It does not mean, therefore, her sinless life, perpetual virginity or the miraculous conception of Our Divine Lord by the power of the Holy Ghost.
From the Catechism of the Catholic Church:
The Immaculate Conception
490 To become the mother of the Savior, Mary "was enriched by God with gifts appropriate to such a role." The angel Gabriel at the moment of the annunciation salutes her as "full of grace". In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God's grace.
491 Through the centuries the Church has become ever more aware that Mary, "full of grace" through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.
492 The "splendor of an entirely unique holiness" by which Mary is "enriched from the first instant of her conception" comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son". The Father blessed Mary more than any other created person "in Christ with every spiritual blessing in the heavenly places" and chose her "in Christ before the foundation of the world, to be holy and blameless before him in love".
493 The Fathers of the Eastern tradition call the Mother of God "the All-Holy" (Panagia), and celebrate her as "free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature". By the grace of God Mary remained free of every personal sin her whole life long.
The Immaculate Conception does not imply that Mary did not need redemption through her Most Holy Son, Jesus Christ.
The Immaculate Conception is an Article of Faith among Catholics.
Good answers all around, but let me add what the Blessed Virgin Mary herself said to St. Bernadette at Lourdes only a few years after the proclamation of the dogma:
Que soi era immaculada concepcion
I am the Immaculate Conception
So, maybe the question shouldn't be what is the Immaculate Conception but who is the Immaculate Conception.
Holiness and immaculate conception of Mother Mary is a default necessity which directly stems from the purpose of God to save mankind from eternal death.
God had a plan to save his prized creation from the time of Adam’s disobedience in Garden of Eden. That’s the reason God became man in Jesus Christ. In order that we might not suffer the penalty of our evil choices (sin), He, like a loving father, paid the penalty for our sins. He allowed his only begotten Son, Jesus Christ, to be born as human being, live an earthly life propagating His message of salvation.
God allowed his Son to be murdered on a Roman cross. But this act of great evil gave rise to an even better state of affairs, and the greatest act of love in the universe: paying the penalty for the wrong choices we make, which were the result of the way He created us in the first place! In the cross of Christ He has provided a full pardon from the consequences of the evil in our lives.
For this purpose to fulfill, Jesus was to be brought in this world with a human nature and hence needed a human mother. But there was a problem. A Son who is Divine in nature cannot come in a womb of a women who is defiled by the very nature as human beings.
Precisely because of this reason, God at the instance of conception in St Anna’s womb, Blessed Virgin Mary was preserved free from all stain of Original Sin. This was done by a singular grace and privilege granted by Almighty God, in view of the merits (Divinity) of Jesus Christ, the Saviour of the human race “.
Mary, rather than being cleansed after sin, was completely prevented from contracting Original Sin in view of the foreseen merits of Jesus Christ, the Saviour of the human race.
This we say is the “Immaculate Conception” of Virgin Mary. That is she was sinless at the time of her birth.
In Luke 1:47, Mary proclaims:
"My spirit has rejoiced in God my Saviour."
This is referred to as Mary's pre-redemption by Christ.
One important point of belief of nearly every Christian fellowship and denomination is that Jesus Christ is the perfect sacrifice for our sins, and that this perfection is part of the reason His single sacrifice is able to cover the sins of so many. If Jesus were not perfect, his death would have been inadequate, and we are all lost.
This causes a small problem for those groups that believe in original sin: the idea that we inherit the guilt of Adam and Eve when they ate of the fruit of the Tree of Knowledge of Good and Evil. If Jesus also inherits this sin, he would no longer be the required perfect sacrifice.
Some such groups deal with this by concluding that this guilt is only passed down through the paternal line. Since Jesus had no biological father, he escapes original sin.
Catholics and some related groups believe that guilt is passed down both paternally and maternally. It's a side note, but this is probably a better interpretation, as Eve ate the fruit first... if you accept original sin at all (ie: Ezekiel 18:20). The Immaculate Conception refers to how Jesus escapes the guilt of his mother.
The idea of the Immaculate Conception is that Mary received special grace from the moment of her conception, such that at the time of her birth (and even through her entire lifetime) she was free from the burden of sin. This does not have to specifically mean that she never herself sinned (though many do believe this); it can also mean that she was in a state of special grace granted by God, sufficient to free Jesus from the original sin that He would have inherited from Mary. The important point is that when someone talks about Immaculate Conception, they're actually talking about Mary, and the reason why is the need for Jesus to not inherit original sin from his earthly mother.
It's also worth noting that there is no record, prophecy, or allusion to the Immaculate Conception anywhere in the Bible. The nearest is the "full of grace" expression in Luke ch. 1. However, the same phrase is used in a number of other places in scripture (Acts 6:8 and Colossians 4:6 for a start) for less-exalted purposes.