In general terms, terrestrial planets are characterized by rocky or metallic compositions and cores. It is known that the earth has a core of mostly iron and nickel. This calculation was made using the measurable force of Earth's gravity, which relates directly to the density of its composite materials. It is believed that the other terrestrial planets, which happen to be those nearest earth, have similar compositions. While the core of the earth is primarily composed of iron, the outer surfaces are composed mostly of rocks like basalt, sodium and granite.
This similarity was caused by the formation of the planets from the protoplanetary disk. A protoplanetary disk was a disk-shaped cloud of matter. Stars are formed in these. It is believed that the terrestrial planets formed nearer to the center of this disk, where the sun formed. Because of their formation so near the hot center of the disk, the terrestrial planets were too gravitationally weak to hold much water or many of the lighter gases together. The heavier elements, then, became the foundation of these planets.
On the outer edges of the protoplanetary disk, the newly forming sun's gravity was not so strongly felt. It was here that liquids and the lighter gases were drawn to one another by gravity to eventually form the jovian planets. This theory is borne out by the fact that the jovian planets are largely composed of these materials. The jovian planets are composed almost entirely of hydrogen and helium. They do not have solid surfaces, and their cores are far smaller and denser than those of the terrestrial planets.
There are further differences between the terrestrial and Jovian planets. The Jovian planets, for instance, do not share the near-perfect spherical shape of the terrestrial planets. This is partially due to their lighter composition, but the Jovian planets also spin far more rapidly than their terrestrial counterparts. For example, despite the fact that some 990 earths could fit inside of Jupiter, Jupiter spins on its axis more than twice before the earth completes its 24-hour rotation. A day on Jupiter lasts roughly ten hours.
A second difference is in the composition of their atmospheres. The Jovian planets have atmospheres of mostly hydrogen and helium. The terrestrial planets, except Mercury, have atmospheres composed of mostly carbon dioxide and nitrogen. Mercury is too small and too near the sun to have any atmosphere. Jovian planets also have more moons than their terrestrial counterparts.