The Apostles' Creed was written by a group of Christians who wanted to provide those of other faiths a chance to be baptized, and it gave them this list of "rules" to follow. The earliest record of the Creed's existence came about in a letter written in 390 A.D. by the Council of Milan.
Many people are still under the assumption that the Apostles' Creed was written by the 12 Apostles, but theologians now know this to be false. The Apostles' Creed is slightly different in every translation. The most common one known in the United States is the English Language Liturgical Consultation version. It covers in detail what the Apostles believed as far as the death and resurrection of Jesus Christ, the virgin birth, Jesus' role as a judge, the role the church is expected to play and the role of the Trinity.
The Apostles' Creed provided a link between Christian faith and tradition, helped different churches accept one another and provided a church that followed the rules outlined in the creed. Those people who wanted to convert to Christianity had to be baptized to finish the journey. They memorized the creed and lived by it before and after their baptism. This made it easy for people to convert because they didn't have to know everything about their new religion, just the creed.