In long exposure photographs, the bright trajectories of moving objects become visible elements. The lights of the cars become bright lines, the stars form paths in the sky and the water softens more (silk effect). In this way only the bright objects will form visible traces and, on the contrary, the dark objects will disappear. Ships during the day with long exposure will disappear, but with their lights in the night will form bright stripes.
While there is no clear definition of what constitutes "long", the intention is to create a photograph that shows the effect of the passage of time either through softened waters or streaks of light. A 30-minute photo of a static and surrounding object can not be distinguished from a short exposure, therefore, the inclusion of movement is the main factor to add intrigue to long exposure photos. Images with exposure times of several minutes also tend to cause the movement of people or dark objects to disappear (because they are anywhere for only a fraction of the exposure time), often adding a serene and other-worldly appearance to long exhibition photos.
When a scene includes both fixed and mobile subjects (for example, a fixed street and moving cars or a camera inside a car that shows a fixed panel and moving landscape), a slow shutter speed can produce interesting effects, such as light trails
Long exposures are easier to achieve in low light conditions, but can be made brighter by using neutral density filters or specially designed cameras.
We leave you with a wonderful collection of photographs of long exposure examples compiled by the magazine Bored Panda.