Imagine a stream flowing out of the mountains into the ocean. As the water flows quickly downhill, it carries all kinds of sediment particles, from mud to sand sized grains, to larger gravel and pebbles. As the stream enters the ocean, the water slows down and deposits the sediment. The heaviest sediment grains (gravel and pebbles) drop out first and the lightest (mud), settles to the floor of the ocean further offshore. A major factor to classify clastic sedimentary rocks is the size of the sediment.

Organisms that live in environments where sediments are formed will die. Their remains accumulate and are incorporated into rocks. When you find fossils in rocks they are referred to as fossiliferous sedimentary rocks.

Particles move into the open ocean or lake from a river
and settle by size and density. 

Gravel along a river bed