A christening is when a child is named.
At Baptism someone becomes a member of the Body of Christ.
Baptism happens with water and spirit. For some people during this ceremony they will be given the gifts of the Holy Spirit which supports a believer in their walk as a Christian and also supports the members of the church/the church itself.
Jesus stated that, "Truly, truly, I tell you, unless one is born of water and the Spirit, he cannot enter the kingdom of God” (John 3:5) (Acts 16 31-34).
We are born with our spirit which also contains the Spirit of God (Psalm 104-30) "Thy sendest forth thy spirit, they are created." From the Father- " But know that the Lord hath set apart him that is godly for himself: the Lord will hear when I call unto him" (Psalm 4). Being Baptised with more of the Holy Spirit at Baptism (which Jesus laid the way for) allows one to grow in the Spirit and become more fully ourselves emulating the attributes of God. This is the presence of God with us here and now and is a third attribute of God.
Not everyone will recieve gifts of the holy spirit during baptism and some may recieve them later in life.
I was baptised at 7-8 and it was a simple service and didn't involve full immersion of water.
Most church's now offer some classes you have to attend in order to be baptised so they make sure you understand what you are doing.
Not everyone is baptised as a child.
Some people are Christians who have not yet been baptised and they can be walked with in their journey to baptism and indeed they are Christians too if they believe.