Flagella are composed of three parts. The basal body, the hook, and the filament. The basal body is the base of the flagella, and it is attached to the actual cell. The hook is attached to the basal body. The hook is the portion of the flagella that creates a spinning "propellor" motion, allowing the bacteria to be motile. The filament is the longest and thinnest part. It is attached to the hook, and the filament is the part that spins due to the movement of the hook.
Bacteria can be classified by their number of flagella, in conjunction with the location of the flagella on the bacteria. This is otherwise known as the arrangement of flagella. The types are:
Monotrichous bacteria, they have one flagella attached to one side of the cell.
Lophotrichous bacteria have several flagella attached to one side of the cell. This creates a "squidlike" appearance.
Amphitrichous bacteria have several flagella at both ends of the cell.
Lastly, peritrichous bacteria have flagella randomly arranged on the surface.