The Solar System isn't flat! We often compare the orbits of other celestial bodies to the orbit of the Earth. The plane on which the Earth orbits is referred to as the Ecliptic.
Most of the planets within our solar system have orbits that are within 2 degrees of this plane. The notable exceptions are Mercury with an angle of 6 degrees to the Ecliptic and Pluto (yeah I know it's not a planet) with wild angle of 17 degrees! Pluto also has a highly elliptical orbit.