What is the Immaculate Conception? 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:


Mary's freedom from any personal sin
Mary's perpetual virginity
The virgin birth of Jesus
The virginal conception of Jesus (mentioned above) 




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.