The prefix "uni" means "one," so unicellular organisms have only one cell. Most kinds of bacteria are examples of unicellular organisms. Within that single cell, all functions needed for life are carried out.
Multicellular organisms have more than one cell, as indicated by the prefix "multi," which means "many." Most living organisms, both plants and animals, are multicellular, with specialized groups of cells that carry out specific functions required to sustain life in the organism. Some cells will be differentiated to fulfill respiratory functions, some will handle distribution of nourishment and removal of wastes throughout the organism, some will enable the organism to respond to external stimuli.