The inner and outer planets vary at the most basic level -- the compounds with which they are made. Inner planets are often called "rocky planets" because they are made of solid materials while the outer planets are made up almost entirely of gas.
Inner planets are significantly smaller than those that rest farther from the sun. The smallest inner planet, Mercury, measures 3,031 miles in diameter while the largest outer planet, Jupiter, measures 88,846 miles in diameter. This size difference is due to the laws of gravitational attraction. Were the inner planets larger, their mass would cause the Earth to pull them in instead of allowing them to orbit.
While inner planets spin relatively slowly, outer plants spin quickly. The difference in speed is so noticeable that, were you on Jupiter, your day would be nine hours and 55 minutes long, while if you were on Venus, you'd have to wait 243 hours for the equivalent of an Earth day to pass.
Inner planets, including Earth, have few moons, if any at all. Outer planets, on the other hand, are often paired with many natural satellites. These outer planets have so many moons, in fact, that counting them is difficult for scientists.
Logically, the planets that form the inner group take less time to orbit the sun because they are closer. Outer planets take significantly longer as they have more ground to cover. Jupiter, for example, takes 164 Earth years to make a complete orbit around the sun.