While meditation is viewed as communication from source, or observation of source, during prayer we generally speak to source. There are many different types of prayer, some very formal and ritualized: "Our Father who art in heaven...," while others are informal: "Yay, God!" Some prayers have a beseeching quality: "Dear God, please help me...," while others are affirmations: "Mother/Father God, I see myself healed and well..." Some prayers employ bargaining tactics: "If you make my wife stay I promise to...," and others are pure expressions of gratitude: "Praise the Lord!" Sometimes we ask for information or a sign from above.
Many people believe that we shouldn't "bother" God with petty concerns because God is (or should be) busy taking care of real problems: "I'll find my own parking space; you stop the war in Iraq." Others believe that source can be everywhere at once, and that the more we connect with it, the better.
Some people are more comfortable addressing their prayers to angels, ascended masters, ancestors, or other divine representatives because these figures feel more accessible to them: "Angels, surround me now!"
While there is no right or wrong way to pray, I personally have found the most benefit from speaking freely, but respectfully, with God, angels, etc., often throughout the day. I find that the more I engage with them, the more they engage with me, and the more I feel connected with the divine. I ask for help whenever I need it, express all of my feelings and concerns, surrender whatever feels overwhelming, and offer gratitude regularly. In fact, my two favorite little prayers are: "I surrender this to you, God," and "Thank you, God!"