Rocks are classified into three categories on the basis of their mode of formation. They are: 

• Igneous Rocks — solidified from magma and lava; 
• Sedimentary Rocks — the result of deposition of fragments of rocks by exogenous processes; 
• Metamorphic Rocks — formed out of existing rocks undergoing recrystallisation.

Further Reading:

• The Rock Cycle
• Geography Notes