The most common feature of sedimentary rocks is that they are organized in layers of different composition piled on top of each other. Any layer of rock in a sedimentary sequence that can be distinguished from the layers above and below is a bedding plane. Bedding planes, also called beds or strata (singular: stratum) are the simplest sedimentary structures and the smaller lithostratigraphic unit used by geologists to describe sedimentary formations. What allows the differentiations of a bed from the surrounding ones are differences in lithology (e.g. alternations of limestone and shale) or grain size (e.g. alternations of sandstone and conglomerate). In geology each bed represents an event, i.e. a period of time in which the physical processes active in a sedimentary basin allowed the deposition of a specific sediment. An ‘event’ in geology can be a long period of time, for example the settling of clay particles on the seabed over thousands of years, but also a fast process, like the rapid deposition of a submarine turbidity current. Bedding planes deposit by gravity and are influenced by other physical processes like currents that distribute the sediments within the basin. The combined effect of these phenomena spreads the sediments laterally and horizontally, in some cases for hundreds of square kilometers, towards the margins of the sedimentary basin, producing horizontal with very limited thickness and wide lateral extension. 

The order of beds in an undisturbed sedimentary sequence (i.e. when the sequence was not tilted/folded by tectonic processes) follows the order of deposition, with the younger strata on top and the older at the bottom (Principle of Superimposition). Each bed is unique and reflects an instant of the history of a sedimentary basin when its sedimentary input, energy, and eventual life forms (fossils) remained constant. A sequence of beds can, hence, be seen as the sequence of events that occurred over geologic time scales in a basin.

Laminations
In a sequence of sedimentary rocks, bedding represent the first-order lithological alternations that occurs at the scale of more than a cm (centimeters to meters). Subtler and less pronounced planar lithological alternations within beds, by convention at a scale smaller than the centimeter, are laminations or laminae (singular: lamina). Laminations are the smaller units in which a sedimentary sequence can be divided. Laminations are important, because their geometry provides information on the processes (e.g. currents) that were active during the deposition of beds.