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A line segment is a straight line with two endpoints. A line segment is said to be closed if the set of all its points contains its endpoints, open if not, and half-open if only one of the end points is contained. Line segments are also refered to as segments when talking about lines. Note: Segment can also refer to part of a circle.
Congruence is the relationship between two shapes of being the same size and shape regardless of position, mirroring, or orientation. By this definition, two line segments are said to be congruent if they are the same length as any two lines will always be the same shape.