Prayer is generally thought of as being a communication between the person praying and the God or the Absolute that the person is addressing. Prayer is a time of conversation, an opportunity for the person to express needs and request guidance or assistance from the Power being addressed.
Meditation usually means attempting to clear the mind of all active thoughts. Meditation is often aided through focusing the mind on a single item, thought, sound, or image. In this sense, meditation is very different from prayer because meditation does not involve active involvement in forming varied thoughts addressed to a superior being.
Meditation may be linked to prayer, in that one may empty one's mind in a spirit of meditation in response to prayer, opening the mind to receive messages or insights in answer to prayed questions or requests.