Highlands and maria characterize the Moon's surface. Because the Moon has no atmosphere with which to destroy much of the incoming debris from outer space, such as meteorites and asteroids, the Moon's surface is riddled with craters, highlands, and maria. If you have ever taken a good look at the Moon, you will notice that it has a white or gray-colored surface mottled with darker spots. Much of these dark spots are maria. The light-colored parts of the Moon are either craters or lunar highlands.
Perhaps you have heard of the phrase "The man on the moon." The shape of the maria can look like a face. The singular form of maria is mare, which means "sea" in Latin. Observers in the seventeenth century gave the maria areas on the Moon romantic names, and these first observers thought that the dark areas on the Moon were bodies of water. Modern astronomers still use the seventeenth century names of those maria, but they also now know that the dark areas on the Moon are not, in fact, bodies of water but the remnants of lava flows. If you think about the volcanoes on the islands of Hawa'ii that are continuously erupting and solidifying, you will be able to imagine what maria are like.
Astronomers believe that maria are young compared to the rest of the Moon's surface, mostly because the maria do not seem to have as many craters as other areas on the Moon. Maria are also larger than craters (though they are both circular in shape). Scientists believe that this is due to large asteroids hitting the Moon and forming very large craters, and then being filled with lava from the Moon's interior. This theory also explains the large ridges that surround maria.
Lunar highlands, on the other hand, also called terrae, are the light colored parts of the Moon. Again, early astronomers believed that terrae were the "drier" areas of the Moon. About 85% of the Moon's surface is covered by lunar highlands. The lunar highlands have more craters, so they are older than maria. The side of the Moon that faces away from the Earth, often called "the dark side of the Moon," is mainly covered with lunar highlands. This may mean that the crust on this side is thicker. If this is so, this may explain why there are no maria on this side of the Moon. When an asteroid or meteorite hits this thicker crust and forms a crater, the impact isn't strong enough to allow lava to flow through the crust.
The Moon's surface tells us much about its history. By studying lunar highlands and maria closely, we may be able to connect the Moon's history to our own.
Freedman, Roger A. "Universe." Chapter 10. New York: W. H. Freeman & Company, 2008.