A group of rocks formed by solidification from a molten state. Rocks which were cooled quickly are termed volcanic or extrusive, those which were cooled slowly are described as plutonic or intrusive.
Rocks formed from the freezing of a melt (magma). Those that flowed or erupted onto the Earth's surface are termed extrusive while those that never made it (froze within the earth) are called intrusive.
Igneous Rocks - Rocks that are formed from molten or partially molten material.