There are a variety of forms of meditation; relaxation, void, focal, energetic, self-exploration, and contemplative being some of them.
Relaxation meditation is probably the easiest form of meditation and is often used as a lead into the other forms of meditation by first relaxing one's physical body and mind and then by putting oneself into at least a shallow trance state. This is operated by mental direction over the physical body and is often accompanied by breathing exercises.
Void meditation is an extended blanking of the mind, in which one attempts to have no thought and no awareness in order to clear ones mind and open oneself up to new understanding.
Focal meditation is a deep concentration of the mind, in which one attempts to focus all thought and awareness on a single thing, either an object or a concept.
Energetic meditation involves circulating energy through internal systems, primarily for health and development of the "energy body (or bodies)".
Self-Exploration meditation is an internal psychological evaluation of one's life as they've lived it until the present, focusing on past events and personal decisions made that have led the person to where they are today. The purpose of this meditation is so that a person can understand who they are and how a variety of internal and external forces have shaped them into who they are, which can be transferred into understanding how that person can change themself in the future to take on the traits, characteristics, attitudes, etc that they desire to possess.
Contemplative meditation is similar to the focal and self-exploration meditations except that the purpose is to fully understand a single concept, or single group of linked concepts, by way of fully examining all of the concept's various facets.
- Focal Meditation - This Veritas Article describes focal meditation and an exercise for practicing it.
- Stanford University - Links to meditations