Trigger Guns, also known as spray guns, are a necessary part of the pressure wash system.  Early systems did not have a trigger gun. A hose attached to piece of pipe with a nozzle on the end was all you had.  Fortunately for operators the early pressure washer did not have much power.  I guess you could put a brick or something heavy to hold the pipe down while you went to shut off the machine.  Imagine doing that with 4 GPM of water coming out of the nozzle at 3000 PSI!  Hence the invention of the trigger gun.

Generally trigger guns come with a brass 3/8 inch female (F) NPT (national pipe thread) inlet for mounting the hose.  A wand is attached to the 1/4 inch FNPT outlet. Hook a gun up to pressure. Pull the trigger to open the gun to start the flow and release the trigger to close it and stop the flow. 

Trigger gun operation is quite simple.  The handle operates a ball and spring.  When the trigger is released the spring holds the ball against a seat, which shuts the water off.  When the trigger is pulled the spring relaxes allowing the pressurized water to push the ball aside, which in turn allows water to flow.  The trigger gun is designed as a dead man's trigger so if the wand is dropped the water flow stops.  Otherwise, with the power of machines today the wand would fly around and do serious damage, if not kill someone.  Many operators find ingenious ways to  relieve the fatigue of holding the trigger for extended periods of time by using zap straps or tennis balls to hold the trigger open.  Not only is this practice dangerous it is illegal in most jurisdictions.  For a few dollars more you can get an easy pull style gun, which uses water pressure to assist in the operation of the valve inside the gun.

There are many different styles of trigger guns available 

Pistol Style
This is the most common of all the guns.  Virtually all off the shelf pressure washers. that are supplied with a trigger gun, offers this type.  Pistol style guns offer a very comfortable angle for most pressure washing jobs including ground work, walls and fences, around equipment parts and overhead.  Constant overhead work can be hard on the wrist if doing it a lot.

Straight Through Guns
Also known as Linear Guns.  These guns are usually used for continuous overhead work.  The angle that the hose comes into the gun is the same as the angle that the wand comes out of the gun.  This style also works well for cleaning walls.

Front Entry Guns
The front entry gun has the hose coming into the gun ahead of the trigger, just below where the wand is mounted.  These guns are usually of lower quality and are supplies with low cost hobby pressure washer units for home use.

Weep Guns
Weep Guns, also known a Dump Guns are designed to allow a small amount of water through when the trigger is released.  This allows operation of a pressure washer without an unloader valve.  

Open Guns
Open Guns do not require the pump to have an unloader valve as they have no trigger at all.  The only way to shut the water off is at the source.  Open guns are used in combination steam applications where shutting off the flow of steam would build up dangerous levels of pressure and heat.  

Dump Guns and open guns can be operated without unloader valves because they don’t stop water flow.  These guns are found in combination steam application where shutting off the steam flow would have dire consequences.

Trigger guns are a maintenance item and should be repaired or replaced periodically.  Most manufacturers offer repair kits but at the low cost of a new gun it seems prohibitive to spend money on a repair kit.  If a gun leaks when the trigger is released it's time to repair or replace it.  That small amount of leaking water will allow the pressure to drop slightly causing the unloader to jump to replace the pressure.  This constant cycling is hard on the pump and unloader valve.