Here's a breakdown:

* Grain Size: The size of the individual particles (clasts) that make up the rock. This is the most fundamental classification criteria.

* Composition: The mineral composition of the clasts. While less important than grain size, it can help further differentiate rocks.

* Texture: This refers to how the grains are arranged, their shape, and their degree of sorting and rounding.

* Cement: The material that binds the grains together.

Here's how grain size is used to classify clastic rocks:

* Conglomerate/Breccia: Rocks composed of rounded or angular clasts, respectively, larger than 2 millimeters in diameter.

* Sandstone: Rocks composed of sand-sized particles (between 2 mm and 1/16 mm).

* Siltstone: Rocks composed of silt-sized particles (between 1/16 mm and 1/256 mm).

* Mudstone/Claystone: Rocks composed of clay-sized particles (less than 1/256 mm).

Keep in mind that these classifications are based on the dominant grain size. A rock can contain a mix of grain sizes, but the dominant size determines its primary classification.