Rocks can be found everywhere on Earth, as a matter of fact, the Earth’s outer layer (crust) is made up mostly of rocks. Rocks can be found; at the bottom of the oceans, below the earth surface, above the ground and some, even forming mountains. Rocks are a naturally occurring solid aggregate of minerals and, sometimes, organic materials. Rocks are classified by mineral and chemical composition, the texture of the constituent particles and also by the processes that formed them.

Rocks can be classified into three main groups which are; Igneous Rock, Sedimentary Rocks and Metamorphic Rocks. These three main group of rocks are classified by Geologists based on how they are formed. Below, these three types of rocks are explained along with the differences between each of them.

Igneous Rock
Igneous rock originates from the cooling and solidification of molten rock. Igneous rocks are formed either underground or above ground. Underground, the molted rock is called magma. As the magma cool slowly underground, the magma becomes intrusive igneous rocks. On the other hand, Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. Magma above the earth is called lava. Extrusive igneous rocks are formed as the lava cools above ground.

The composition of lava and magma and their rate of cooling (which is responsible for the size of the crystal), determine the minerals that are formed.

Examples:
Large crystal of quartz.
When quartz combined with other minerals it forms into a intrusive rock called granite.
Lava that flow out on to the surface of the earth and makes up a large area of the ocean basin forms extrusive igneous rock is called basal.
Others are;
Pumice
obsidian

Location:
where volcanoes have or do exist

Characteristic:
crystalline, glossy, coarse-grained

Sedimentary Rocks
Sedimentary rocks are rocks which are formed when layers of sediment (clay, sand, gravel and minerals) settle to the bottom of the ocean and then compacted (pressed together by the weight of additional deposit) and a cementing process by the chemical action of water and certain minerals (over thousands of years) cause sedimentary rock to form.

Examples:
Among the important varieties of sedimentary rock, distinguished both by texture and by chemical composition, are conglomerate, sandstone, tillite, sedimentary breccia, strata, shale, marl, chalk, limestone, bituminous coal, lignite, gypsum, and rock salt.
Location:
Where oceans or bodies of water once existed or still exist.

Characteristic:
Soft layered, may contain fossils, footprints, raindrop impressions, concretions, oolites, ripple marks, rill marks, and cross bedding.

Metamorphic Rocks
Metamorphic Rocks are rocks that have "morphed" into another kind of rock. They are formed from igneous and sedimentary rocks lying deep below the Earth's surface which are subjected to high temperatures (heat), high pressures, or chemical reaction. This cause the rocks' crystal structure to change.

Examples:
Limestone becomes marble
Shale becomes slate
Granite becomes gneiss
Sandstones (some) become quartzite

Location:
Deep within the earth (where pressure and heat have a very large affect)

Characteristic:
Hard, may contain bands or layers, may contain crystals