When a garage door service company gets a phone call in reference to a broken spring, the first thing the homeowner usually says is “the cables are broken”. This makes sense because the cables on a garage door will fly every which way, fall to the ground, become disconnected from the door, or even get caught in between the garage door and the jamb. It is rare for cables to break or need replacing when a garage door spring breaks. If your garage door has two torsion springs, the second spring will keep the cables tight and on the drums. You will have to look up at the springs to determine if one is broken.
Extension springs are long, lighter-weight springs that run perpendicular to the door and are mounted above the horizontal portions of the door tracks. These springs are tensioned by stretching out, using cables and pulleys, as with the torsion system. Because extension springs are merely suspended between two brackets (they are not mounted to a rod, like torsion springs), they must have a safety cable running through each spring. This helps contain the spring in the event of a breakage. Without the safety cable, a spring breaking under tension is a very serious safety hazard. If you have old springs that don't have safety cables, you should install them even if you aren't replacing the springs.
No matter what kind of door you are installing, be very careful. The instructions that follow are merely guidelines. You should read your garage door manufacturer's instructions and owner's manual very carefully before attempting either type of installation. And if you're a do-it-yourself greenhorn, skip this project and hire an experienced professional. It may be more expensive, but garage door installation is on the higher scale of DIY difficulty.