A small scar on the arm, courtesy Wikipedia Commons
A scar is the result when large quantities of tissue are damaged, but manage to heal themselves. Scar tissue is visually different from normal tissue and can be distinguished merely by visual examination, although it will often come up in radiological examinations as well.
When tissue grows normally, it grows in a more or less criss-cross pattern. This is what is seen as "normal" tissue. However, when tissue repairs itself, it grows all in one direction and it is this inconsistency which is seen as a scar, similar to what is seen in, for example, a run in a stocking.
Scars on the skin are usually the result of severe cuts, although surgical cuts cause the worst scars. However, internal tissue can scar as well. Scarring of the internal organs can be caused by a number of factors, such as drug abuse or exposure to toxins. Scarring of the liver results in cirrhosis. Scarring of the lungs results in pulmonary fibrosis.