1. Igneous rocks: Formed from the cooling and solidification of molten rock (magma or lava).

* Intrusive igneous rocks: Form when magma cools and solidifies beneath the Earth's surface. Examples: granite, gabbro.

* Extrusive igneous rocks: Form when lava erupts onto the Earth's surface and cools. Examples: basalt, rhyolite.

2. Sedimentary rocks: Formed from the accumulation and cementation of sediments, which are fragments of pre-existing rocks, minerals, or organic matter.

* Clastic sedimentary rocks: Formed from fragments of other rocks. Examples: sandstone, shale, conglomerate.

* Chemical sedimentary rocks: Formed from the precipitation of minerals from solutions. Examples: limestone, rock salt, gypsum.

* Organic sedimentary rocks: Formed from the accumulation of organic matter. Examples: coal, oil shale.

3. Metamorphic rocks: Formed from the transformation of existing rocks (igneous, sedimentary, or even other metamorphic rocks) under intense heat and pressure.

* Foliated metamorphic rocks: Have a layered or banded appearance due to the alignment of mineral grains. Examples: slate, schist, gneiss.

* Non-foliated metamorphic rocks: Do not have a layered appearance. Examples: marble, quartzite.

These are the three main types of rocks based on how they form. Remember that rocks can go through multiple cycles of transformation. A sedimentary rock can be metamorphosed into a metamorphic rock, and a metamorphic rock can be melted and then re-solidify as an igneous rock.