These layers can consist of sediments or, more typically, of rock. Often this is sedimentary rock, though strata as used in relative dating also can include layers formed from volcanic ash or indeed from lava and pyroclastic flows. Note that "Stratum" is the singular form of strata.
Above is the geological use of the term strata, though described from a biological perspective. Two additional meanings of the term are found exclusively in biology and these are strata as layers of tissues found in organisms and strata as layers of vegetation found in ecosystems.