The following extracts are from wikipedia.
Vipassana (Pali) or vipasyana (Sanskrit)in the Buddhist tradition means insight into the nature of reality. A regular practitioner of Vipassana is known as a Vipassi (or vipasyin). Vipassana is one of the world's most ancient techniques of sitting meditation, the inception of which is attributed to Gautama Buddha. It is a practice of self-transformation through self-observation and introspection to the extent that sitting with a steadfast mind becomes an active experience of change and impermanence. In English, vipassana meditation is often referred to simply as "insight meditation".
In a broader sense, vipassana has often been used as one of two poles for the categorization of types of Buddhist meditation, the other being samatha (in Pali or Sanskrit). Samatha is a focusing, pacifying and calming meditation, common to many traditions in the world, notably yoga. It is used as a preparation for vipassana, pacifying the mind and strengthening the concentration in order to allow the work of insight. In Buddhist practice it is said that, while samatha can calm the mind, only insight can reveal how the mind was disturbed to start with, which leads to prajña (wisdom) and jñana (knowledge) and thus to understanding; preventing the mind from being disturbed again.
Vipassana as practiced in the Theravada tradition includes contemplating Buddhist teachings, such as the Four Noble Truths, as well as more experiential forms such as body awareness. In the latter forms it is a simple technique which depends on direct experience and observation. It can be related to the three trainings taught by the Buddha as the basis of a spiritual path: adherence to a sila (abstinence from killing, stealing, lying, sexual misconduct and intoxication), which is not an end in itself but a requirement for the second part, concentration of the mind (samadhi). With this concentrated mind, the third training, in the context of this technique is detached observation of the reality of the mind and body from moment to moment.
In the experiential forms, meditation consists of the experiential observation of mind and matter in their aspects of impermanence, unsatisfactoriness and lack of an inherent, independent essence or self.