The process in which a relatively unspecialized hemopoietic precursor cell acquires the specialized features of a leukocyte. A leukocyte is an achromatic cell of the myeloid or lymphoid lineages capable of ameboid movement, found in blood or other tissue. [ http://purl.obolibrary.org/obo/CL_0000738 http://www.ncbi.nlm.nih.gov/pubmed/16551264 ]