They're detected using the red/blue shift in the star caused by the planet orbiting it. All planets exert gravational force on their star, just like the star exerts gravational force on the planets. The difference is the star is much much bigger than the planet. Consider this: Jupiter appears to orbit the sun, but they both really orbit a point a few miles just outside the corona of the sun (the center of mass). Earth does orbit the sun, but only because the center of mass is a few feet from the center of the sun.
If I were at Alpha-Centuari A, looking back at Sol, I would see Sol move in a sinisudial pattern, matching the period of Jupiter's orbit, with a slight wobble to that because of Saturn. (We would not be able to detect the wobble caused by the other planets, because the equipment and techniques are not that refined.)
For more information, and a more-precise explanation, see Exoplanets.org