+ Are Catholics Christians? +
Most non-Catholic Christian denominations accept Catholics as Christians. A very few do not.
A dictionary would say that a Christian is someone professing belief in Jesus as Christ or following the religion based on the life and teachings of Jesus.
Catholics would fit this definition.
In the Nicene creed, from 325 C.E., Catholics profess:
We believe in one Lord, Jesus Christ, the only Son of God, eternally begotten of the Father, God from God, Light from Light, true God from true God, begotten, not made, one in being with the Father.
Through Him all things were made.
For us and our salvation He came down from heaven: by the power of the Holy Spirit, He was born of the Virgin Mary, and became man.
For our sake He was crucified under Pontius Pilate; He suffered, died, and was buried.
On the third day He rose again in fulfillment of the scriptures: He ascended into heaven and is seated at the right hand of the Father.
He will come again in glory to judge the living and the dead, and his kingdom will have no end.
We accept Jesus Christ as our Lord and Savior. We are baptized as Jesus commanded in Matthew 28:19, "in the name of the Father, and of the Son, and of the Holy Spirit."
We truly are spiritually "born again," we just don't usually use those words.
For a complete description of what Catholics believe, see the Catechism of the Catholic Church: http://www.usccb.org/catechism/text/index.htm
+ Holy Tradition +
I have much more to tell you, but you cannot bear it now. But when he comes, the Spirit of truth, he will guide you to all truth. He will not speak on his own, but he will speak what he hears, and will declare to you the things that are coming. (John 16:12-13)
The Catholic Church does not use Holy Scripture as the only basis of doctrine. It could not. The early Catholic church existed before and during the time that the New Testament was written (by Catholics).
There were hundreds of Christian writings during the first and second centuries. Which New Testament writings would become official was not fully decided until about 400 C.E.
Catholics believe that the Holy Spirit was guiding the early church (and is guiding the church today) to make the correct choices about things like:
+ The Holy Trinity (which is also only hinted at in the Bible)
+ Going to church on Sunday instead of Saturday (which is actually directly against one of the Ten Commandments)
+ The Communion of Saints
+ Which writings include in the New Testament?
Things that are even more modern like
+ Slavery is bad. Slavery is never declared evil in the Bible. This was one of the justifications for slavery in the Confederate States.
+ Democracy is good. The Bible states that either God should be the leader of the nation like Israel before the kings or kings should be the leader, "Give to Caesar that which is Caesar's." This was talked about a lot during the American Revolution.
This second source of doctrine is called Apostolic Tradition.
Do Christians who do not allow the continuing guiding force of the Holy Spirit to make their beliefs more and more perfect, still endorse slavery as Colossians 3:22 commands, "Slaves, obey your human masters in everything"?
Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours. (2 Thessalonians 2:15)
We instruct you, brothers, in the name of (our) Lord Jesus Christ,to shun any brother who conducts himself in a disorderly way and not according to the tradition they received from us. (2 Thessalonians 3:6)
I praise you because you remember me in everything and hold fast to the traditions, just as I handed them on to you. (1 Corinthians 11:2)
http://www.usccb.org/catechism/text/pt1sect1chpt2.htm#80
+ With love in Christ.