To speak of meditation and visualization is to speak of a topic so vast that generalization is almost impossible. Mystics and monks, athletes and warriors, therapists and medical professionals all have their own particular slant on meditation and visualization and how they should be accomplished. Yet a few general principles are true across the board: meditation and visualization are mental exercises with some characteristics in common and some differences.
Meditation is a state of deep concentration. It brings the mind to a focus, a "one-pointedness." In doing so, it blocks out or quiets the typical mental chatter most people constantly carry around in their mind. Buddhists speak of the "monkey mind," the mind that darts from topic to topic. Meditation is giving the monkey a banana, giving it one focus to keep it from hopping from one thing to the next. That focus can be the breath, the sensations of the body, a mantra or a sound. Or the focus can be a static or moving image.
When the focus of a meditation is an image, the meditation becomes visualization. Visualization is a specific kind of meditation. Visualization is sometimes called mental imagery or mental rehearsal. It can take the form of a visual or kinesthetic view. If you are using a kinesthetic focus, you create in your mind the experience of doing something. You might feel the sensations in your body. You might experience the action and its consequences in great detail in your mind. If you are performing a simple visualization, you picture a setting, another person or a sequence of events -- something outside yourself. Visualization can be both visual and kinesthetic. You can visualize yourself in a setting, experiencing the impact of that setting on your body and mind.
Meditation that is not Visualization
Though visualization is typically a specific kind of meditation, not all meditation involves visualization. Mantra meditation, for example, involves bring the mind to focus on a word or phrase, which you repeat over and over again either aloud or in your mind. Meditation on the breath involves keeping the mind focused on the sensation of the breath as it enters and leaves the body. Both of these meditations try to wean the mind from the steady stream of images that cross it. The purpose of these meditations is to stop visualization.
Visualization meditation, however, cultivates mental imagery. The purposes of these visualizations are a wide-ranging as the mental images themselves. Visualization for relaxation might include a mental image of a relaxing place--a natural landscape or a place that feels safe and secure. Visualization for healing might include mental pictures of the body attacking a disease or of a healing light bathing the body. Visualization for religious purposes might include mental images of important religious figures or of heaven or hell. Visualization for athletic training purposes is typically kinesthetic and involves seeing and feeling the body perform a sport-specific movement perfectly.