What is a Movie Extra?

So what does a movie extra do in a movie or on television? The movie extras provide the rich tapestry of background performers in almost every scene. Regardless of the scene, location or time period, movie extras are used play the role of background fillers. You will see them as crowds at sporting events, students in the school hallway, patients in hospitals and diners at restaurants. Movie extras are the soldiers on the battle field and even the criminals in jail cells.
What makes movie extras different from actors, is by definition, movie extras never speak. When a movie extra is given dialogue, even if it's just one word, then the extra becomes an 'Actor'.