What is the difference between cellulitis and sepsis?

Big difference: Cellulitis is just infection of skin. Sepsis is if that infection goes to blood and produce chemicals affecting other organs.

Sepsis: Cellulitis is infection of the skin. Sepsis is a clinical syndrome defined by having 2 or more of the defining symptoms of fever (or hypothermia), tachycardia, tachypnea, and leukocytosis or leukopenia. Septicemia means having multiplying bacteria in your bloodstream.