There are two common differences; the ends and the pitch. On an extension spring, the end coils are typically turned perpendicular to the body to create either hooks or loops. A compression spring will have either plain, closed or closed and ground ends. The other main difference is that extension springs are tight wound; that is without spacing between the coils. While in service, an extension spring’s coils will begin to pull apart (or extend). As with other types of springs, both extension and compression springs have limitless applications.