Class I (light-duty) 2,000 lbs. Max. Gross Trailer Weight (GTW) & 200 lbs. Tongue Weight (TW).
Receiver hitch uses 1-1/4" sq. mounts with 1/2" dia. receiver pin.

Class II (medium-duty) 3,500 lbs. GTW & 350 lbs. TW.
Receiver hitch uses 1-1/4" sq. mounts with 1/2" dia. receiver pin.

Class III (heavy-duty) 5,000 lbs. GTW & 500 lbs. TW.
With Weight Distribution Configuration₁: 10,000 lbs. GTW & 1,000 lbs. TW.
Receiver hitch uses 2" square mounts with 5/8" dia. receiver pin.

Note 1: Some "Class III" hitches are listed at lower GTW capacity; The distinction for class III here seems to be the 2" receiver as opposed to 1-1/4". See Hidden_hitch Rating Codes below.
Read the label on your hitch. I got a Draw-Tite Max_e_Loader (41531) which is listed at 6,000 lbs. on their web site, but the label on the hitch listed it at 3,500 lbs.

Note 2: If you are going to use an accessory such as a bike carrier or cargo rack you may want a Class III hitch to get the 2" receiver which will provide more stability even if you don't need the towing capacity.

Class IV 8,000 lbs. GTW & 800 lbs. TW.
With Weight Distribution Configuration: 12,000 lbs. GTW & 1,200 lbs. TW.

Class V 15,000 lbs. GTW & 1,500 lbs. TW.
With Weight Distribution Configuration: 16,000 lbs. GTW & 1,600 lbs. TW.
receiver hitch uses 2" square mounts with 5/8" dia. receiver pin.

Hidden-Hitch Rating Codes for Class III hitches

GTW - Gross Towing/Trailer Weight, TW - tongue weight
GVWR - Gross Vehicle Weight Rating (same as GTW or GTWR)

Note: Some hitches are not rated to support Weight Distribution.

1. A "weight distributing hitch" has extra spring bars to transfer some of the weight load to the front of your vehicle.

See also: About Weight Distribution and Trailer Hitch Weight Distrubition at etrailer.com

Trailer Towing Guidelines:
Most trailer problems I have heard of are caused by the ball connection; Either the ball breaking off, comming loose or the hitch separating from the ball.
Always make sure the ball's weight rating is greater than the gross trailer weight rating. As for the hole diameter, it must be less than 1/16" greater than the ball shank diameter. When tightening, always use the lock washer and make sure a portion of the ball shank extends past the bottom the nut.

The tongue weight on the vehicle's hitch should be within 10-15 percent of the total (fully loaded) trailer weight. These specifications should never be exceeded and can lead to dangerously unstable driving conditions along with excessive wear and tear on your vehicle.

Towing capacity, Gross Towing/Trailer Weight (GTW), is a combination of the engine and transmission in your vehicle plus things like the wheelbase. A jeep Wrangler with short wheelbase has a lower GTW because of its shorter wheelbase. (See jeep page).

Tips:

• Weight Distribution: Amount of a trailer's weight that rests on the tow vehicle; should be 10 to 15 percent with conventional trailers, 15 to 20 percent for fifth-wheels.
  Move heavier items to the front of the trailer to avoid sway.
• Speed: Maximum speed is 55 or 60 MPH. See your trailer manual and state vehicle codes. e.g. Calif., Others
• Following distance: Allow double the amount of space between you and the vehicle in front of you when towing a trailer.
• Braking: Remember the extra weight will increase your stopping distance.
• Curves: Avoid applying brakes when going around a curve; The trailer can push the back of your vehicle out and cause a jackknife. Instead break before the curve and accelerate in the curve so you pull the trailer thru the curve rather than it pushing you.
• Turns: Take wider turns to avoid hitting the curb.
• Hills: Use lower gears for climbing and descending grades to avoid unnecessary transmission shifting and overheating brakes.
• Sway: At the first signs of trailer sway, slow down by taking your foot off the accelerator. Let vehicle speed decrease but do not put your foot on the brake pedal, which can make the situation worse. Keep the steering wheel steady; Don't try to steer out of a sway. Once you're down to a safe speed, carefully apply the brakes and drive slower. If the trailer contues to sway, stop and adjust your load.
  Sway may also be caused by wind or gusts from passing traffic if you have a trailer with a large surface area e.g. a travel trailer. In this case there are sway control devices you can add to your hitch.

Safety Checklist:

• Maintenance Checklist (Up to date)
• Hitch Ball Tight
• Hitch Ball Lubricated
• Hitch Secured in Receiver
• Safety Chains Crossed (too act as a cradle for the tongue if the ball breaks) and Attached with enough slack
• Coupler Latched onto Ball
• Load Distributed Correctly and Securely
• Trailer Level when Hooked Up
• Trailer Lights Working Correctly
• Lug Nuts Checked and Tightened
• Inspect Tires for Cuts
• Tire Pressure Checked
• Breakaway Battery Charged
• Breakaway Cable Hooked Up
• Pin or Bolt Through Coupler Latch
• Check coupler tightness, lights and safety chains after 50 miles
• Block Tires When Loading or Unloading

See Also:
RVs and Trailers