Technically, only the Earth travels in the ecliptic plane. It's called that because it's the plane where eclipses happen, where the Moon passes directly between the Earth and the Sun -- which, naturally, would correspond to the plane of the Earth's motion around the Sun. Naturally the Moon doesn't orbit exactly in the ecliptic (it's off by about 5 degrees), or there'd be a solar eclipse every four weeks and a lunar eclipse midway between each one. And all the other planets' orbital planes
range from about 3/4 of a degree (Uranus) to over 7 degrees (Mercury) off of the ecliptic.