From Academic Kids
Rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks are classified by mineral and chemical composition; the texture of the constituent particles; and also by the processes that formed them. Rocks are thus separated into igneous, sedimentary, and metamorphic rocks.
Igneous rocks are formed from molten magma, and are divided into two main categories: Plutonic rock and Volcanic rock. Plutonic rocks results when the magma cools and crystallises slowly within the Earth's crust, while Volcanic rocks result for the magma reaching the surface either as lava or fragmental ejecta.
Sedimentary rocks are formed by deposition of either detrital or organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation. The latter can occur at or near the earth's surface, especially in the case of carbonate-rich sediments.
Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. The metamorphic-rock-forming PT conditions are always higher than those at the earth's surface, and are sufficiently high enough to change the original mineralogy to either other minerals or to other forms of the same minerals (e.g. by recrystallisation).
Classification of Igneous Rocks (http://www.geol.lsu.edu/henry/Geology3041/2IgneousClassify/IgneousClassFlow.htm)