Rocks are formed from one or more minerals.  They are classified by how they were formed into three groups:  sedimentary, igneous, and metamorphic.  

Ingenious Igneous 
Igneous rocks are formed from molten (melted) rock. Underneath the earth's surface, melted rock is called magma.  Rocks which form under the earth's crust are intrusive rocks.  Granite is formed in this way.  Molten rock above the earth's crust is called lava.  Rocks which form above the earth's crust are extrusive rocks.  The basalt columns below were formed by lava which slowly cooled.  

Sedimentary, my dear Watson 

Sedimentary rocks are made of pieces of older rocks that have been broken by wind and rain.  The gravel, sand, and mud wash to the bottom of streams, lakes and oceans.  Over time, these particles build up many layers, and the bottom layers consolidate into rock. 

Metamorphic Rocks 

Metamorphic rocks are formed from other rocks through physical and chemical changes resulting from heat and pressure.  

New rocks are constantly being made and old rocks are constantly being destroyed.  This is called the rock cycle.