I believe everyone prays, even the atheist. We all say words of hope that some being will intervene in our lives. When we are faced with a major decision, we may do all the comparisons of positive and negative points to arrive at our conclusion, but often we will ask that inner man what those points are. This is a prayer for guidance. We cannot always come up with all of them on our own. When we are faced with illness, we hope within ourselves that it will not be too bad, and that we will heal quickly. This is a prayer for healing. Even though we may not be petitioning God, we are asking some entity to intervene.
To put it simply, prayer is communicating with that entity. Most of us call that entity God. Prayer is asking something of God, hoping that He hears and understands our prayer, and then listening for and accepting the response. Prayer is our faith-in-action. Prayer can be in the form of a conversation or can be a statement of faith. It takes on many forms and all can be effective.