Can anybody explain why orbit of comets is so different from those of Planets?

Because comets are objects from the Kuiper Belt or the Oort Cloud that are knocked out of their normally (roughly) circular orbits in the outer Solar System into highly elliptical orbits in the inner Solar System.

All orbits are conic sections, parabolas, ellipses (including circles), or half of an hyperbola. They all have different eccentricities. Circles have 0.00000000 eccentricity. Ellipses eccentricity < 1.00000000000000000000000: parabola eccentricity = 1.000000000000000000000000000000. hyperbola eccentricity > 1.0000000000000000000000000000000000000. Comets have elliptical, parabolic, and hyperbolic orbits. Look up the mathematical definitions of ellipses, parabolas, and hyperbolas.

If a body in a multi-body planetary system has a circular orbit, over time, the masses of and gravitational effects will alter a circular orbit into an elliptical orbit, and a body with small mass approaching a a body with a large mass CNN have it's orbit changed from an elliptical orbit to a parabolic or hyperbolic orbit and escape from the Solar system. The missing mass in the asteroid Belt that went into making a planet that was torn apart by Jupiter was thrown out of the Solar System over 4.5 billion years or more.

Comets are perturbed from the Oort cloud mainly in any direction as the solar system journeys around the Milky Way and comes nearby to another star's gravity. Or they could have been interstellar objects picked up in the solar system's journey.

http://www.nasa.gov/mission_pages/stardust/mission...