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Is the process of manipulating and re-arranging video shots to create a new work. Editing is one part of the post production process; other post production tasks include titling, colour correction, sound mixing, etc.
Most videos serve a purpose such as telling a story or providing information. Editing is a crucial step in making sure the video flows in a way which achieves this goal. Give the video a particular "angle". The final product can be tailored to support a particular view point, impart a message or serve an agenda.