Why Is Christianity Considered A Monotheistic Religion, When Christians Believe In The "Trinity" ?

No, the Trinity does not mean God has a split personality.  The Trinity is actually all the same Person- God.  But think of it this way.  A man can be a son, a dad, and a husband.  He can be all  3 things, but still be the same person.  The Trinity is God the Father, God the Son, and God the Holy Spirit.  It can be hard to explain and understand, but that is where faith comes in.  If we knew and understood everything about God, then He would not be much of a God!  But Jesus is God in the flesh, and the Holy Spirit is God Whose presence is all around us, and lives inside those who have accepted Jesus as their Savior.  
We are monotheistc, we believe that God is One God, and there is no other.  But He came to earth in the form of Jesus to die for us, and He rose again.  When He went back to Heaven, He sent the Holy Spirit to us to help us.
My question is... Do you know Jesus as your Savior?

I find it hard to take your question seriously since your name is "Raunchy Man," but oh well, here goes...the trinity of the Bible is: Father, Son, and Holy Spirit. This means that God is these three things at once. So, yes, we Christians are monotheists. We don't believe in MULTIPLE gods (like the god of the sea, or the goddess of wisdom, or anything), we just believe in the Lord almighty, the one and only, Jesus Christ, the savior. However, God plays three roles in the Bible. So, to wrap things up and answer your question: Christianity is a monotheistic religion.

The Trinity is one.  God, Jesus, and the holy ghost - making up one.  Here we have the father (God), the son (Jesus) who represents God's earthly form and the Holy Ghost which is the eternal love that flows between them.  So, the father and the son are one and the love is part of the equation - thus we have one entity.  It is a trinity:  Father, Son, and Holy Ghost when broken down but it represents a whole.  Christians worship one god therefore it is monotheistic, but the trinity is part of the religion itself.  One should not confuse individual parts of the religion which are translated in the literal way with the entire religion and what it really means.