Bacteria can be classified based on the location and number of flagella on their cell surface. There are six categories: atrichous bacteria have no flagella; monotrichous bacteria have a single flagellum at one end; cephalotrichous bacteria have several flagella at one end; amphitrichous bacteria have one flagellum at each end; peritrichous bacteria have flagella all over their body; and lophotrichous bacteria have two or more flagella at both poles. Flagella are hair-like protein structures that protrude from the cell wall and aid bacterial locomotion in liquids.