how are clastic rocks classify?

Answer 1

By size:

>2 mm, pebble, cobble, etc. (conglomerate, breccia)
2-.0625 mm, sand (sandstone)
<.0625, clay and silt (shale, mudstone)

Answer 2

Taking QFL 24-7's answer a bit farther, once the classification into conglomerate, sandstone, siltstone, or shale is established, geologists use various minor components to classify even further. For example, a quartz arenite is a sandstone composed of almost nothing but quartz and chert. Add rock fragments or another mineral and the rock becomes a lithic arenite or an arkose. As far as I know, due to the economic (oil and gas) important of sandstone, it is the only clastic for which a classification scheme is widely used. And, there are other such schemes apart from the one I mentioned.