How do igneous rocks form?
Always remember that the word igneous is derived from the latin word called ignis which means fire. Igneous rocks are formed either inside the surface of earth or outside the surface of earth. When magma does not get a chance to come out of the surface of the earth it start's cooling there which takes alot of time and leads to the formation of rocks which are called intrusive igneous rocks. These rocks generally have large crystals because they get enough time to grow inside the surface of the earth.
In the other case when the magma gets a chance to come out of the surface of the earth in the form of lava another kind of igneous rocks are formed by the cooling of lava in few days or weeks.These kind of rocks are called extrusive igneous rocks. These rocks either do not have crystals or have very small crystals because the lava get's cooled down very quickly outside the surface of the earth and this restricts the growth of the crystals. Hence, very few or no crystals are found extrusive igneous rocks.