There are two types of cargo trailer tires: bias ply and radial.  Bias tires are often used on entry level trailers and utility trailers.  These tires offer a lower purchase price while sacrificing long term wear and performance.  A very popular upgrade option are radial tires.  Advantages of radial tires include better steering characteristics and better durability , which increases gas mileage.

The Radial design prevents the bottom of the tire (where the rubber meets the road) from deforming, unlike the biasply tire, which does deform under load. 

Additional benefits of a radial tire vs bias ply are:

1) Softer and quieter ride

2) Better gas mileage while towing

3) Flatter, wider footprint for longer tire life

4) Runs cooler than a bias ply, minimizing risk of a blowout on the highway

5) Improved sway control

You can see a wide variety of trailers for sale at www.TrailersPlus.com