The rocks brought back to Earth from the moon fall into four categories: mare basalts, impact breccias, pristine highland rocks and regolith. Basalts are igneous rocks formed by cooled lava deposits. Breccias are formed of smaller pieces of other rocks that are fused together by meteorite impacts. Pristine highland rocks are fragments of the original lunar crust. Regolith is a sand-like soil consisting of particles formed through repeated meteor impacts.
In many ways, lunar rocks resemble Earth rocks, giving credence to the theory that the moon was broken off of the Earth by a massive impact event early in the planet's history. However, moon rocks tend to have fewer volatile compounds and none of the hydrated minerals common in mineral deposits on Earth. Moon rocks are high in iron content, and some have substantial amounts of titanium. Regolith can contain fragments of the other three types of moon rock as well as other loose minerals.
In total, the Apollo missions brought back approximately 842 pounds of rocks from the moon for study. Russian lunar probes brought back another 0.7 pounds of samples, and approximately 106 pounds of lunar meteorites have been recovered from Earth. These are fragments blown off the moon's surface by meteorite impacts strong enough to send the debris into Earth's gravity well.