Sometimes people get the idea that being Catholic is just about rules. While it is true that there are lots of rules for living as a Catholic, it is important to know why these rules exist. Rules for the sake of rules doesn't help anyone. One of the points that Jesus made over and over again during his public ministry was that we have to get to the heart of the rules for them to be helpful.
In the end, Catholicism has rules because Catholicism is a family. All families have rules. Some rules just help to make everyday life run more smoothly (Joe does the dishes on Monday, Sally does the dishes on Tuesday), and others determine whether someone is able to continue living with the family.
The Church has several basic rules, beyond living according to God's law (10 Commandments, etc.), which we have traditionally called precepts, and which lay out the basics of being a Catholic. A Catholic who doesn't follow these is not considered to be in good standing with the Church. It is important to remember that these represent an absolute minimum of Catholic practice- we should strive for a much higher ideal- being saints!
The Precepts of the Church