The Catholic Church does not recognize Mormon baptism as valid because, although Mormons and Catholics use the same words, those words have completely unrelated meanings for each religion. The Mormon’s very concept of God is infinitely different from that of Christians—even though they call themselves the Church of Jesus Christ of Latter-Day Saints.
Mormons believe that God is only one of many gods who were once men and that each of us in turn can become what God is now. This process of men becoming gods is said to go back infinitely. But of course none of these gods can be infinite if they are multiple and had a beginning and are actually human beings. In Mormons’ view, both Jesus and the Father are what we would call glorified creatures.
They also believe that Jesus came into existence after the Father, and that the Father and the Son are not one in being. Thus, although they use the phrase "the Father, the Son, and the Holy Spirit," in their usage this phrase takes on a meaning that is actually polytheistic and pagan rather than trinitarian.
For an in-depth look at this, see the books Inside Mormonism and When Mormons Call by Isaiah Bennett, available from Catholic Answers. For a shorter but equally incisive take, see Fr. Brian Harrison’s two-part series on Mormonism in the April and May-June 2003 issues of This Rock.