This post is the first installment of a series which will try to answer some of those questions. So who were the Apostles?
The word apostle comes to us from the Greek word apostolos. This word originally meant "messenger or envoy" with the root stello (meaning "I send" or "I depart") taking the prefix apo- (meaning "from"). However, our present understanding of the word is closer to "delegate". So the Apostles can be viewed as delegates sent from Christ to preach and share His message. This holds with our understanding of this group as the primary disciples of Jesus who were his closest followers and became the primary teachers of the early Church.
So that answer what and who the Apostles are, so why are they important? First, the Apostles were the first teachers of the faith. They are the ones who heard the Jesus's message firsthand and went out into the world to preach it. Second, the Roman Catholic Church believes in something called apostolic succession. This means that Jesus commissioned the Apostles as the first bishops and, before they died, the Apostles passed on that role to a new generation of leaders. Consequently, every bishop today can trace his succession back to one of the Apostles. Thus, the bishops we have today draw their authority to teach and confer sacraments from the Apostles themselves.