Mars is one of the most fascinating of the planets, and is the one that we probably know the most about, though there are still enough mysteries to make every new discovery eagerly awaited by the scientific community.
Being the fourth planet out from the sun and laying between the orbits of Earth and Jupiter, Mars is one of Earth's nearest neighbors in space. The diameter of Mars is a little more than half that of the Earth, and it is about half again further away from the sun. The size means that Mars doesn't have nearly as substantial a gravity as the Earth, and because of this, most of the atmosphere that it once had has seeped away into space, leaving a very weak atmosphere of mostly carbon dioxide, as compared to the mostly Nitrogen atmosphere of the Earth. Even with an atmosphere like the one we have, the additional distance from the sun would mean that the temperature on the surface of Mars would be substantially colder than on the Earth. Without the atmosphere, though, the temperature on Mars is far more harsh than anything found on the Earth - from about -190 degrees Fahrenheit (about -123 degrees Celsius) to close to 70 degrees Fahrenheit ( 21 degrees Celsius). It is gets so cold, in fact, that at the poles, carbon dioxide freezes and falls as Dry Ice snow!
Mars has two very small moons, Phobos and Deimos, which are not only so much tinier than our own moon, they are also much closer the the surface and zip around the planet very quickly. It would be possible on the surface of Mars to see these moons set and rise more than once a day. Also, while a "day" on the surface of mars is only very slightly longer than a day on the Earth, it takes Mars almost 687 days to move entirely around the sun (its "year"), mostly because of its distance from the sun.
Mars is home to the largest volcano ever found in the Solar System (Olympus Mons, at almost 88,500 feet high (27,000 meters). It also has the largest canyon so far found in the solar system, Valis Marinaris, which is far deeper, wider, and longer than the Grand Canyon. On the Earth, it would reach nearly all the way across the United States!
While there is a growing amount of evidence of free flowing water on the surface of Mars at some time in the past, it is currently so dry that it isn't uncommon for powerful sandstorms to cover the entire planet for months at a time. Many scientists believe, however, that water ice will be found below the surface.
Currently there are plans in the mix to send a manned expedition to Mars. This will be nowhere near as easy as it might sound, as it will take up to about 6 months for a craft to get there, and they will have to wait for a year and a half (Earth time) to be able to return. Still, considering the many extremely interesting discoveries found out about Mars in just the last few years, since the advent of Mars robotic probes, the potential to find out more very interesting information is even greater with a manned mission. I, for one, am going to very much look forward to the information that is returned from this mission. We may even learn a great deal more about the Earth and how it functions, which could help us all.
If you want to start learning about the solar system, I suggest that Mars might be the best place to start.