The density across a molecular cloud is fragmented and its regions can be generally categorized in clumps and cores. Clumps form the larger substructure of the cloud, having the average size of 1 pc. Clumps are the precursors of star clusters, though not every clump will eventually form stars. Cores are much smaller (by a factor of 10) and have higher densities. Cores are gravitationally bound and go through a collapse during star formation.