What name was used by Peter for Baptism? We know that Jesus gave this command.


  Matthew 28:19 (NIV) 
  
  Therefore go and make
  disciples of all nations, baptizing them in the name of the Father and
  of the Son and of the Holy Spirit,


But later, we find a statement made by Peter.


  Acts 2:38 (NIV) 
  
  Peter replied, “Repent
  and be baptized, every one of you, in the name of Jesus Christ for the
  forgiveness of your sins. And you will receive the gift of the Holy
  Spirit.


Are these two words contradicting each other?
From the Catholic Encyclopedia:


  There has been a theological controversy over the question as to
  whether baptism in the name of Christ only was ever held valid.
  Certain texts in the New Testament have given rise to this difficulty.
  Thus St. Paul (Acts 19) commands some disciples at Ephesus to be
  baptized in Christ's name: "They were baptized in the name of the Lord
  Jesus." In Acts 10, we read that St. Peter ordered others to be
  baptized "in the name of the Lord Jesus Christ". Those who were
  converted by Philip. (Acts 8) "were baptized in the name of Jesus
  Christ", and above all we have the explicit command of the Prince of
  the Apostles: "Be baptized every one of you in the name of Jesus
  Christ, for the remission of your sins (Acts 2).
  
  Owing to these texts some theologians have held that the Apostles
  baptized in the name of Christ only. St. Thomas, St. Bonaventure, and
  Albertus Magnus are invoked as authorities for this opinion, they
  declaring that the Apostles so acted by special dispensation. Other
  writers, as Peter Lombard and Hugh of St. Victor, hold also that such
  baptism would be valid, but say nothing of a dispensation for the
  Apostles. The most probable opinion, however, seems to be that the
  terms "in the name of Jesus", "in the name of Christ", either refer to
  baptism in the faith taught by Christ, or are employed to distinguish
  Christian baptism from that of John the Precursor. It seems altogether
  unlikely that immediately after Christ had solemnly promulgated the
  trinitarian formula of baptism, the Apostles themselves would have
  substituted another. In fact, the words of St. Paul (Acts 19) imply
  quite plainly that they did not. For, when some Christians at Ephesus
  declared that they had never heard of the Holy Ghost, the Apostle
  asks: "In whom then were you baptized?" This text certainly seems to
  declare that St. Paul took it for granted that the Ephesians must have
  heard the name of the Holy Ghost when the sacramental formula of
  baptism was pronounced over them. (Baptism: Form)


In other words, the predominating opinion states baptism in the name of Christ alone is against Christ's command and is therefor unlikely. Rather, "in the name of Jesus Christ" refers to performing the baptism at Christ's command or for Christ's purpose, much like you might claim a piece of land in the name of the Queen. Acts 19 clarifies the ambiguous language, supporting this opinion, by suggesting that baptized people know of the Holy Ghost from having been baptized in Him.