The Maria on the Moon were named by early astronomers, who thought these darkened regions looked like "seas" (in fact, "maria" is Latin for "seas.) They make up about 17% of the Lunar surface, and are lower than the Lunar Highlands. Maria are thought to have formed by volcanic activity in the Moon's distant past. The cause of the volcanism may have been from vigorous asteroid impacts.
Interestingly, the near side of the Moon has almost all of the maria, and most of the iron content, which suggests that the impacts which initiated volcanism were on the near side as well.