Our moon is the fifth largest satellite in the solar system.

Facts about the Earth's moon(as taken from www.usgs.org):
Circumference........6,785 miles
Diameter.............2,160 miles
Surface area.........14,600,000 square miles
Mass.................81,000 trillion tons
Gravity..............1/6th that of the Earth
Mean distance from Earth (between centers)...238,857 miles
Average orbital velocity...2,287 miles per hour
Orbital distance traveled..1,500,000 miles
Period of revolution and rotation..27 days, 7 h, 43 min, 11.5 s
Temperature variation
Lunar "noon" (Full Moon)....243 degrees F
Lunar "midnight" (New Moon)..-279 degrees F

Originally, the moon was much closer to Earth and in fact, there is a theory that the moon and Earth were once joined, but an asteroid (the size of Mars) impact separated the two causing the moon to be spiraling away from Earth at about 2 inches per year.