Sedimentary Rocks:

 are formed by the accumulation and subsequent consolidation of sediments into various types of rock.  The key is the sediments.  Sediments are unconsolidated material and have different origins.  Ultimately, the origin of these sediments is the weathering, erosion and/or the chemical breakdown of other rocks.  These "other" rocks could be igneous, metamorphics or even other sedimentary rocks.  The type and size of the sediments and how they are formed will lead to the classification of the different sedimentary rocks listed below. 

Another common factor to sedimentary rocks is that they originate on the surface of the Earth, unlike most igneous and metamorphics rocks which originate in the interior of the Earth's crust.  Geologists can actually see many sedimentary rocks form or at least see the sediments that will become sedimentary rocks prior to their lithification (literally "turning into stone").  

The lithification of the sediments is usually accomplished by a cementing agent.  Once the sediments are no longer loose sediments, but cemented together grains or crystals, they become a rock.  What happens to sediments from this stage on is called diagenesis. 

Diagenesis is important to study in sedimentary rocks.  It includes the study of the compaction of the rock, physical conditions, chemical alterations and biological interactions.  Cementation is usually the first aspect of diagenesis, but cementation can be episodic, reversed and recementation can occur.  Compression of the rocks can change the banding and increase chemical alterations.  Chemicals can leave or enter a rock through pore space waters, and minerals can crystallize, or dissolve, or become hydrated or oxidized or chemically changed in other ways.  Temperature increases can alter the characteristics of the rock as well.  If too much heat and pressure occur during diagenesis then the rock may wander into the regime of metamorphism.  At times geologist will argue over the boundary between a highly diagenetically altered sedimentary rock and a low grade metamorphic rock.  Diagenesis is extremely important in understanding the history and the resulting effects on the characteristics of sedimentary rocks.