Here's a breakdown:

* Fine-grained rocks have crystals that are too small to be seen with the naked eye. They are often described as having a "sugar-like" or "powdery" texture.

* Coarse-grained rocks have large crystals that are easily visible.

* Medium-grained rocks fall somewhere in between, with crystals that are visible but not very large.

The size of the grains in a rock is determined by the rate at which the magma or lava cooled.