Meditation encompasses an extremely broad array of practices connected to many of the world's religious and philosophical traditions.
Meditation generally includes avoiding (though not harshly) random thought processes and fantasies, and a calming and focusing of the mind. It is not effortful, and can be experienced as just happening. Different practices involve focusing one's attention differently, and a variety of positions and postures including sitting cross-legged, standing, laying down, and walking (sometimes along designated floor patterns).
The stated purpose of meditation varies almost as much as the practices. It has been seen as a means of gaining experiential insight into the nature of reality (religious/spiritual or not), or communing with the Deity/Ultimate Reality. Even without the spiritual aspects, many have gained concentration, awareness, self-discipline and equanimity.
In the samadhi or shamatha, or concentrative, techniques of meditation, the mind is kept closely focused on a particular word, image, sound, person, or idea. This form of meditation is found in Buddhist and Hindu traditions including Yoga, in Medieval Christianity, Jewish Kabbalah, and in some modern metaphysical schools. Related to this method is the method developed by Eknath Easwaran. He called it "passage meditation" -- silent repetition in the mind of memorized inspirational passages from the world's great religions. As Easwaran says, "The slow, sustained concentration on these passages drives them deep into our minds; and whatever we drive deep into consciousness, that we become."
In Vipassana (insight, or seeing things as they are) meditation the mind is trained to notice each perception or thought that passes, but without "stopping" on any one. This is a characteristic form of meditation in Buddhism, especially in some Theravada traditions, and is also a component of zazen, the term for meditation practice in Zen. In at least some forms of vipassana, you do not attend to whatever perceptions arise, but purposely move your attention over your body part by part, checking for perceptions, being aware and equanimous with them, and moving on. This form of meditation has some resemblance with "choiceless awareness" the kind of meditation that Jiddu Krishnamurti talked about.
In annapuna meditation attention is focused on the breath.
See also: Qi, Qigong, Transcendental Meditation, Zen Meditation