The Ecliptic is the apparent path of the Sun through the heavens, from Earth's viewpoint.
The 12 Zodiac Signs correspond to the constellations that the Sun travels through along the ecliptic. There are a few constellations whose boundaries intersect the ecliptic but are not included in the traditional zodiac. The Ophiuchus constellation was recently (2009) suggested by the scientific community as a candidate for inclusion in the astrological zodiac. However, this is not generally taken seriously by astrologers, since the tradition of astrology signs is a system based on far more than the ecliptical constellations. (See my article about Ophiuchus.)
Most of the orbits of the planets and asteroids are close to the ecliptic, within a certain distance of it. However, the orbits of all planets and bodies of our solar system are tilted away from the ecliptic (inclination) to some extent. Some notable planets and asteroids with a high inclination include Pallas Athena (35:03), Pluto (17:08), Ceres (10:37), Mercury (7:00), and Vesta (5:34).
How far a planet is above or below the ecliptic is called Celestial Latitude. This is similar but different from Declination, which measures a planet's position relative to the Celestial Equator rather than the ecliptic.
The Ecliptic is the baseline measurement for Celestial Latitude, which is similar but completely different from the Celestial Equator which is the baseline measurement for Declination.