Christianity is not a set of rules and regulations, nor is it merely a philosophical framework. The word Christian, which means "little Christ," describes a person who has become like Christ. What this means, and why it is necessary, explain Christianity.
We will start with the necessity of being like Christ, since that is where the story starts. "In the beginning God created the heavens and the earth." Thus begins the Bible, which is the written word of God put on paper by various people He chose to record His thoughts. God created man and woman, and all the rest of the natural world, as an act of love. He truly loved His creation, and they loved him as well. However, the first man and woman chose to deliberately rebel against God, and so began a gradual process of shutting Him out.
Ever since then, each person has been born with both an awareness of his or her need for a close relationship with God and a realization of the separation that exists between them. In addition, each of us has done things that we know are wrong. Perhaps it was a lie or an act of cruelty to another person or any of a number of actions that we knew perfectly well were not acceptable. Christians call this wrong behavior "sin", but no matter what you call it, the outcome is the same. God is perfect and cannot accept our sin, so we must rid ourselves of its effects on our lives before we can have that close relationship with Him that we inwardly long for. Unfortunately, nothing that we do on our own can totally erase the effects of these actions. Each one causes us to lose touch more and more with God, and to slip further away from the beauty and perfection that he wants us to experience.
God could, of course, force us to conform to His plans and to experience that beauty by not allowing us to make wrong choices. However, He loves us as individuals and wants us to freely make the choice to love Him back. He already had prepared another way for us to break away from the results of our own wrong actions: He sent His son, Jesus Christ, to come to Earth and be born as a human child, to grow up without ever doing anything that was wrong, and to be killed because of it.
This may seem like a strange plan, and from our perspective I suppose it is. However, God's universe, which He created and we live in, has certain laws. One of those laws is that bad acts must be punished. Obviously, all of us have done bad acts in the past, so we all must be punished. Unfortunately, none of us could stand the punishment we have earned. Jesus died as a way of providing a substitute punishment for us. If we accept his punishment as our own, God will willingly forgive us and take us into His arms as His very own children. This is what He has been waiting for ever since the moment of creation.
This is where we get to the second part of our original question: what does it mean to be like Christ? Acknowledging that we have violated God's law, that we deserve to be punished and can never earn forgiveness on our own, and accepting the death of Jesus as payment for our own sins is essentially what makes a person a Christian. Once we accept that sacrifice, God begins treating us as though we had become as perfect as Christ. He knows, of course, that we are not, but He treats us as though we were. God treats us as though we were as perfect as Christ, but He doesn't stop there. He also sends the Holy Spirit, the third part of what Christians call the Trinity, to help us as we struggle to overcome the years of bad habits and wrong choices that we have been accumulating.
The Holy Spirit helps us in a variety of ways. Sometimes He acts as an aid to our conscience, pointing out that we really should pay more attention to one area of our behavior or another. Sometimes He gives us special abilities, like the ability to speak eloquently or to understand what another person is feeling. Everything the Holy Spirit does, though, is designed to help us and the people we come in contact with to develop a deeper relationship with God. Essentially, he is helping us to become just like Christ.
This, in a nutshell, is the Christian faith. Of course, many aspects of it can be explained in much greater detail, and some parts have been omitted here in the interests of brevity. The sections below will help to elaborate on these concepts.