The diagram above illustrates a process called the rock cycle which describes how rocks are formed.
The first stage (1) is weathering and erosion. Existing rocks are attacked by the rain and wind and small pieces break off and are transported as sediment by rivers, streams and the wind. The sediment is deposited (2) in low lying areas as layers. As the layers build up the heat and pressure increases and over time the sediment solidifies into sedimentary rock (3).
When tectonic plates (the movable plates making up the Earth's crust) collide rocks of all types can be buried very deeply. The heat and pressure at these depths can be enough to cause the rocks to recrystallise which produces a range of metamorphic rocks (4). The type of metamorphic rock produced depends on the parent rock (the rock that recrystallised).
At high temperatures some minerals in metamorphic rocks melt and form magma which is molten rock (5). This magma can rise slowly in the Earth's crust and form magma chambers which sometimes cool slowly, producing granite rock, or can erupt at the surface in volcanoes (6). The volcanic rocks which cool from the lava are quickly weathered and eroded (stage 1) and the rock cycle begins again.

Image reproduced by kind permission of John Reynolds and the Earth Science Teachers' Association.