Flagella are long protrusions on the back of a cell such as the tail of the sperm cell. They can propel a cell through liquid or fluid mediums via their undulation and fluctuation. Eukaryotic flagella are elongated versions of cilia whereas bacterial flagella are completely different both in structure and mechansims.
Bacterial flagella are made entirely of protein and have a diameter of around 12-30nm. Several thousand protein subunit molecules of Flagellin make up one bacterial flagellum, of these molecules there are two types. Three types of flagella arrangement are understood to occur: monotrichous, peritrichous and lophotrichous. Monotrichous flagellum are single and Polar, a peritrichous arrangement occurs with flagella over the entire cell surface, whilst lophotrichous arrangements occur when there are many polar flagella.