There are two types of cargo trailer tires: bias ply and radial.  Bias tires are often used on economy trailers and utility trailers.  These tires offer a cheaper purchase price while sacrificing long term wear and performance.  A very popular upgrade option are radial tires.  Advantages of radial tires include longer tread life and less rolling resistance, which  improves gas mileage.

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

Additional benefits of a radial tire vs bias ply are:

1) Softer and smoother

2) Better gas mileage while towing

3) Flatter, wider footprint for better tire wear

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