Igneous rocks are formed when magma crystallizes and solidifies. After this transition from liquid to solid, igneous rocks are classified as either intrusive or extrusive.
In order for an igneous rock to be considered intrusive it must undergo the change from a liquid to a solid state inside of the Earth. Due to the high temperatures inside the Earth, this process can take thousands or even millions of years. This type of igneous rock tends to be much larger and has a coarse texture.
Extrusive rocks are usually found near volcanoes or fissures in the Earth's crust where magma has erupted. These tend to be much smaller and have a very smooth texture. Obsidian is one of the most well-known, and also one of the most common of the extrusive igneous rocks.