Trailer Hitch, Bumper Hitch, or Bolt On Receiver. 

  Hitches are broken into "Class" ratings.  Each "Class" dictating a different weight carrying capability.  Weight and Class ratings are stamped or have a sticker on the hitch and usually consist of the following four numbers:  Maximum Weight Carrying - the recommended maximum trailer Gross Vehicle Weight (GVW) if towing with a standard receiver and ball assembly.  Maximum Weight Distributing - the recommended maximum trailer GVW if using a weight distribution hitch (discussed in #4),  Maximum Tongue Load Weight Carrying, - maximum recommended amount of weight that the trailer can place on the ball mount or bumper area, and Maximum Tongue Load Weight Distributing - maximum amount of weight that the trailer can place on the ball mount area if using a weight distribution hitch.

Receiver or Ball Mount. 

This unit slides into the square receiving tube of a hitch.  Items to consider: receivers also have weight ratings associated with them, drop heights vary, the hitch ball shank diameter
size, and slide-in tube size can also vary. Shown in the picture at right are several different weight rated receivers ranging from 4000 - 9000 lb, all happen to have 2" square slide-in tubes which is standard for most Class III hitches,  the drop heights ranging from a 2" drop, also called a straight (top), to an 8" drop at the bottom.
Note: Typically on the front plate or drop down portion of the receiver will be stamped with the recommended hitch pin (see item #5) and hitch ball shank diameter sizes (see item #4).

Recommended Trailer Towing Height.

  This is a good time to address this issue, it seems to be a point that folks are not aware of and it is very important.  All trailer manufacturers have a recommended towing height for their trailers.  Usually the rule of thumb is level to slightly up in the nose, but NEVER down. An exaggerated nose up attitude or nose down can cause stability problems.   To find out weather your trailer is sitting properly, you can attach it to your vehicle, leave it empty and find a level piece of ground, then stand back and judge for yourself.  Why I bring this up now  is because the drop height of your receiver is what can fix or create this problem.  All vehicles set a varying heights off the ground. Most pickup trucks will use either a 2"(straight) receiver to a 4" or even a 6" for some taller 4wds.  One the other hand, we have found that most SUVs will use either a 2" (straight) or a 2" or 4" flipped over to be used as a rise instead of a drop.  The point  to be stressed here is that it is important for your trailer to be setting properly when being towed, if not stability can be adversely affected.  Please check with your trailer manufacturer for their recommended tow heights and follow their suggestions.  Don't forget to ask how to take the measurement, i.e. with trailer empty and connected and setting level &/or measure to the base of the ball as opposed to the top etc...

Hitch Balls 

Again a couple of items to consider.  Like everything else they too come with different weight ratings. Ball Diameter sizes differ ranging  from 1 7/8" to 2" and up  trailers.  Shank length and diameter also vary. Shown left to right:  a) 2" 5000 lb ball - 1" diameter shank  with a one inch riser built in. b) 2" 7000 lb ball - 1" diameter shank. c) 2 5/16" 12,500 lb ball - 1-1/4" diameter shank. About the riser ball (item a), this can help with your recommended trailer towing height.  If you are just slightly off such that a 2" shorter receiver will make you sit too high this type of ball may be just the ticket.

Hitch Pins.  

These pins hold the receiver in place in the hitch.   hitch pins generally come in two different   If you look at the receiver, most of them have their hitch pin and ball shank size requirements stamped on the face or drop portion of the receiver.   Funny thing about hitch pins, is that when you look at them they do not appear to be very substantial.  Yet when you put a hitch together it seems to be the only thing item holding the receiver in place, so why isn't it more substantial and weight rated like everything else?   

 Weight Distribution Hitch .

(often mistaken for sway bars or sway control device) 

A weight distribution hitch slides into your standard hitch or bolt on receiver (item #1) in place of a regular  receiver and ball assembly. The "V" shaped arms, called spring arms, run parallel underneath the crossbars or tow bars of your trailer.   The two items shown between the spring arms are called cuffs.  These cuffs attach to the tow bars of your trailer.  When you hook your trailer up, the chains at the end of the spring arms connect to the cuff which is usually directly above it.   On the cuff there is a type of rocker switch if you will, when that rocker switch is engaged the chains tighten and the truck and trailer basically lift at the hitch point, transferring weight to the axles of truck and trailer.  To help you understand it a bit better think of  the trailer as now being supported or towed from three areas, (ball and two cuffs) as opposed to just one in a normal ball/receiver assembly.

When do I need to use one?

The answer is simple.  If you are hauling a trailer that has a GVW  higher than your hitch's maximum weight carrying recommendation and less than the maximum weight distribution recommendation.  If your trailer's GVW is higher than the latter you need to get a new hitch.  Also, there may be instances where your trailer's GVW is equal to or less than the weight carrying recommendation  BUT, the trailer applies more than 10% of its weight to the tongue. It's possible in that type of situation to exceed the maximum weight carrying tongue load also making necessary the use of a weight distribution hitch.  

Factory Installed Hitches.

Probably 95% of the vehicles sold with factory installed hitches have Class III or 5,000/10,000 lb hitches installed.  In many cases the vehicles themselves are rated to tow in excess of 7000 lbs.    If you are going to purchase a vehicle and planning to pull a larger trailer, check into this.  You may find that the your vehicle's manufacturer requires the use of a weight distribution hitch above certain weights. However, most times we find that is not the case and the 5,000/10,000 lb hitch is applied because it is part of the "standard" tow package. 

 Sway Control Device.

Many times this name is mistakenly  used to describe the weight distribution unit.     A small hitch type ball is mounted to either the left or right of the main trailer hitch ball, likewise a matching ball is mounted on the trailer tow bars. The balls are then connected with a two part rod that has an inner and outer sleeve, each sleeve being attached to a ball.  As the trailer goes around the corner, the outer sleeves slides over the inner sleeve shortening the rod and allowing the trailer to turn.  Under  driving conditions, it is friction that causes the rods not to slide in an attempt to keep the trailer from swaying.   These  units have a "friction adjuster" on them, so if you have a trailer that has a large tendency to sway you can set it higher and vice-versa.