Vipassana in pali means clear seeing, another word to describe it is discernment. It is an insight meditation which through powerful concentration accurately and without illusions discerns the true nature of things.
It differs from Shamatha meditation in that with Shamatha we direct the mind to an object (usually the breath) in meditation and work in stages to achieve single pointed focus. Vipassana is more about being with whatever arises without doing anything with it. Merely observing and noting, feeling, thoughts etc. It is has the qualities of penetrative, investigation that focuses more on the phenomena aspect which reveals the true nature of reality.
This style of meditation directly helps to pull out the seeds or imprints that are left in our psyche by these our emotional defilements and distorted thoughts so that they will never reoccur. It brings unchanging and everlasting inner peace, joy and harmony.
In the modern times it is also the name given to the 10 day retreat style meditation developed by S. N. Goenka a lay Burmese meditation teacher.
The Buddha practiced these two styles of meditation, first cultivating single pointed focus and then using that powerful attention to deeply penetrate the true nature of reality. It is thought that without having attained some stages of Shamatha meditation, the practice of Vipassana meditation is ineffective.