Long-exposure photography or time-exposure photography involves using a long-duration shutter speed to sharply capture the stationary elements of images while blurring, smearing, or obscuring the moving elements.
The paths of moving light sources become clearly visible.
When a scene includes both stationary and moving subjects (for example, a fixed street and moving cars or a camera within a car showing a fixed dashboard and moving scenery), a slow shutter speed can cause interesting effects, such as light trails.
Long exposures are easiest to accomplish in low-light conditions, but can be done in brighter light using neutral density filters or specially designed cameras. Long-exposure photography is often used in a night-time setting in order to produce a near daytime effect in the photo.
By leaving the camera’s shutter open for an extended period of time, more light is absorbed, creating a brighter product. If the camera is stationary for the entire period of time that the shutter is open, a very vibrant and clear photograph can be produced.
Source : Wikipedia