Flagella are hairlike structures that some bacteria use for locomotion. Depending on their arrangement patterns, specific names have been applied. Some bacteria have one single flagellum (singular) on one end of the cell, they are said to be monotrichous. Some bacteria have flagella on both ends of the cell. These are said to be lophotrichous. Some bacteria have flagella surrounding the cell. These have the flagella in a peritrichous pattern.
Simply put, a flagellum is a hairlike structure that grows out of a cell.