I have heard that, although Catholics use the Bible, their faith is not solely based upon it. I have heard it said that for Catholics, tradition precedes the Bible, and that is why there are some practices and doctrines that the Church has that cannot be found in the Bible. The alleged reasoning behind this is that tradition predates the Bible and it's the tradition that influenced the making of the Bible. I'm not sure about this though. Is it true that the Catholic Church has some beliefs that override the Bible, or are not based upon it? I'm sorry if this sounds stupid, but I don't really have a clue.
Is the Catholic faith purely based on the Bible? No. Is it largely based on the the Bible, and is the Bible the most sacred text in the Catholic faith? Yes.
There are lots of important parts of Catholic theology that are extrabiblical traditions, and generally not believed by Protestant congregations who consider themselves "purely" based on the Bible. Protestants are not "purely" based on the Bible, either. It is entirely possible both churches would be purely based on the Bible, but the Bible doesn't answer every question people like to ask.
Here is one belief that deviates from the Bible in both churches: the virginity of Mary after giving birth to Jesus. If someone believes anything on that subject, they have exceeded the scope of the Bible.
Certainly the Bible says that Mary was a virgin when she conceived, through the time when she gave birth to Jesus (see Matthew 1:23-25). The Bible does not talk about her sex life after that. In Protestant churches, many people believe Mary would have had a normal sex life with her husband Joseph, even going on to bear more children. Evidence: Jesus has siblings. In the Catholic church, the official belief is that Mary was eternally a virgin -- that is, throughout her entire life, she never had sex. Jesus' siblings are explained away by other extrabiblical traditions: perhaps Joseph was a widower, and had children from another wife; perhaps the translation is incorrect, and the words for "brother" or "sister" should truly be translated as "cousin".
Both beliefs are based on things outside of the Bible. Because the Bible doesn't say one way or another.