What is the difference between fine-grained and coarse -grained igneous rock? (Geology)

Igneous rocks are formed through crystallization from melt. All igneous rocks, with the exception of volcanic glass (obsidian), are made up of interlocking crystals. In rocks that cool slowly, deep below the surface of the earth, these crystals can be quite large and visible to the naked eye. Common coarse-crystalline rocks that cooled slowly in a magma chamber are granite or gabbro. In rocks that cool rapidly from lava after a volcanic eruption, these crystals may be too small to be visible with the naked eye, but a hand lens or microscope easily reveals the crystalline nature of these rocks. Basalt is an example of a fine-crystalline rock that cooled rapidly at the surface of the earth. Igneous rocks can be thought of as the ultimate parent material for most soils. The igneous rock may have been ground up and transported innumerable times between when it first solidified and what we see today.