Even though the words baptism and christening are used interchangeably, there is a subtle difference. Christening refers to the naming ceremony (to "christen" means to "give a name to") where as baptism is one of seven sacraments in the Catholic Church.
In the sacrament of Baptism the baby's name is used and mentioned, however it is the rite of claiming the child for Christ and his Church that is celebrated.
Baptism represents a deliberate act of identification with the person of Jesus Christ and his Church. This decision implies a personal relationship with Jesus that requires nurturing through such things as worship, prayer, Bible study and other spiritual disciplines.
edit Trivia about baptism and christening
- It says in the scriptures that Jesus was baptized by John. Jesus told his disciples, just before his Ascension "Go make disciples of the world, baptizing them in the name of the Father and the Son and the Holy Spirit."
- It's a common misconception that the Catholic Church only recognizes its own Baptism as being valid. Catholics actually believe that any Baptism/Christening, regardless of denomination, which uses the words "in the name of the Father, and of the Son and of the Holy Spirit" and involves the use of water, either through pouring or immersion, validly initiates a person as a Christian.
- Baptism is the only sacrament that can be administered by anyone in an emergency (i.e., in danger of death.) Although ordinarily it is administered by a priest or deacon, in an emergency the person performing the Baptism doesn't have to be Catholic or even Christian, so long as the proper form (words) and matter (water) are used.