Scar tissue forms when the process of tissue repair is carried out mainly by the stroma, which is the supporting connective tissue. Here, fibroblasts are involved. Fibroblasts are a type of cell conatined in connective tissue that secrete the molecules that form the matrix, or ground substance and fibers. The matrix determines the tissue's qualities, for example, the matrix of blood is fluid, the matrix of bone is hard.
So, fibroblasts use collagen and other materials from the matrix to make scar tissue, in a process called fibrosis. Because scar tissue is not formed from the types of specialised cells that made up the original tissue, whether it be skin or organ tissue, they have a different appearance and are not able to perform the same functions.
What is critical in repairing tissue to its original state, is the ability of the cells, called parenchymal cells, from the original still working part of the body associated with the injury, to regenerate. For example, other skin cells, or if you've had surgery on say your abdomen, whatever andominal cells were involved. The body has different cells, with specific functions associated with different organs and parts of the body. That is how those organs are able to function in specific ways.