These are the springs that go in the back of your guitar, hooked at one end onto the spring claw and inserted in the holes at the bottom of your bridge sustain block. These provide the tension to return your bridge back into the neutral position after you've pulled on the tremolo bar.
These come in three different tensions - weak is great for people who want to use their tremolos for crazy, severe, whammy tricks. Medium is a great compromise between low and high tension. High tension is great for people who want to do subtle, fluttery effects.
47mm long, they fit virtually all tremolo equipped guitars.