Some great information from one of my favorite science websites; Space.com
Terrestrial planets are Earth-like planets (in Latin, terra means Earth) made up of rocks or metals with a hard surface — making them different from other planets that lack a solid surface. Terrestrial planets also have a molten heavy metal core, few moons, and a variety of topological features like valleys, volcanoes and craters. In our solar system, there are four terrestrial planets, which also happen to be the four closest to the sun: Mercury, Venus, Earth and Mars. During the creation of the solar system, there were likely more terrestrial planetoids, but they likely merged or were destroyed.
Mercury is the smallest terrestrial planet in the solar system, about a third of the size of Earth. It has a thin atmosphere, which causes it to swing between burning and freezing temperatures. Mercury is also a dense planet, composed mostly of iron and nickel with an iron core. Its magnetic field is only about 1 percent that of Earth’s. The surface of Mercury is similar to that of the Moon; it has many deep craters and is covered by a thin layer of tiny particle silicates.
Venus
Venus, which is about the same size as Earth, has a thick toxic atmosphere that traps heat, making it the hottest planet in the solar system. Much of its surface is marked with volcanoes and deep canyons — the biggest of which is 4,000 miles long. Only two spacecraft have ever penetrated Venus’s thick atmosphere. And it’s not just spacecraft that have trouble getting through the atmosphere — there are fewer crater impacts on Venus than other planets, because all but the largest meteors can’t make it through the thick air.
Earth
Of the four terrestrial planets, Earth is the largest, and the only one that has liquid water, which is necessary for life as we know it. Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy metal core. Earth’s atmosphere contains water vapor, which helps to moderate daily temperatures.
Mars
Mars has some of the most interesting terrain of any of the terrestrial planets. The red planet has the largest mountain in the solar system, rising 78,000 feet above the surface. Much of the surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.
Mercury, Venus, Earth, Mars
Beyond the solar system
NASA's Kepler space observatory had discovered more than 2,300 potential alien planets as of April 2012. Using the data from the telescope, scientists have calculated that there may be billions of Earthlike planets in the Milky Way galaxy.
Non-terrestrial planets
Not all planets are terrestrial. Jupiter, Saturn, Uranus and Neptune are gas giants, also known as jovian planets. Gas giants tend to be much bigger than terrestrial planets, and they have thick atmospheres full of hydrogen and helium. On Jupiter and Saturn, hydrogen and helium make up most of the planet, while on Uranus and Neptune, the elements make up just the outer envelope. Landing on a gas giant is nearly impossible, since only the core is solid, and the atmosphere is storm-filled and very thick.
— Katharine Gammon
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