Recall that weathering processes create several types of sedimentary materials.  Mechanical weathering produces only PARTICULATE sediment, but chemical weathering produces both particulate and DISSOLVED sediment -- ions dissolved in the water that weathered their source rock.  For the moment we will focus on particulate sediments, but will return to dissolved sediments when we discuss chemical sedimentary rocks.

Particulate sediments can be of many sizes, from huge boulders to tiny clays.  However, we generally recognize only three or four major categories of sizes, separating them more or less arbitrarily.  Anything larger than 2mm (about the size of a B-B or the thickness of a quarter) is called GRAVEL.  Particles between 2 mm and 1/16th of a mm (about the diameter of a sewing thread) are called SAND.  Particles smaller than this are too small to see, and are often lumped as MUD, with the coarser of these tiny sizes sometimes called SILT and the finer CLAY.

When particulate sediment accumulates in a sedimentary environment it is referred to as DETRITAL sediment.  A rock formed when this lithifies is a DETRITAL SEDIMENTARY ROCK.  Such rocks are further classified based on their grain sizes, shapes, arrangements, and (in some cases) mineralogy.