Prop guns used in movies can be real guns that have been loaded with blank ammo, a highly detailed toy, a rubber replica or what is essentially an air gun.
Blanks
Blank-fire guns are real, working weapons that have been minimally modified to fire blank cartridges. Blank cartridges contain gunpowder, but do not fire a shell or bullet. They are used for scenes that require a believable muzzle-flash and a loud report. They can be costly and dangerous.
"Function" Guns
"Function" guns are essentially highly detailed toy, replica guns. They look and feel like real guns and can be loaded with realistic looking brass bullets, but they do not fire. These are typically used for scenes that require an actor to brandish and manipulate a firearm but not fire it. These are just for show.
Rubber Guns
Rubber guns are just that, fake guns made of rubber. These are used for scenes that require several, non-firing weapons to be present. These are the most cost-effective props, but do not look real in closeup and do not make any noise.
Suicide Guns
Basically the same as blank guns--though not as realistic--but they do not create a muzzle flash. The barrel is sealed and are just used for sound effects.
Weapons Master
A weapon's master works with all the prop guns on a movie set. He is in charge of maintaining, storing and ensuring the guns are safely handled and never manipulated. The weapon's master must handle each prop gun before and after each time the weapon is used.
Brandon Lee
During filming of "The Crow," actor Brandon Lee was fatally wounded by a malfunctioning blank gun. "Dummy" cartridges were made from live bullets by removing the shell, dumping the gunpowder, reinserting the shell and loading the gun with bullets that could not fire but would appear to be real. An unknown member of the crew fired the gun between scenes and a shell made its way into the barrel. Later, the same gun was then loaded with gunpowder-filled blanks and fired at Lee during a scene. The rouge shell was discharged by the force of the blank and lodged in Lee's spine. He died after being rushed to the hospital.