Putting a set of ATV mud tires on your quad makes a big difference if you do much driving in the mud. The stock tires are designed to work reasonably well in most types of off-road conditions, but not the best in any of them. Mud tires are designed for one thing only - to get traction in mud and keep you moving.
Before you buy a set of mud tires, though, there are three things that you should consider. If you don't pay attention to these tips, you could wind up with the wrong tires for your ATV and you'll be no further ahead than with the stock set.
The first thing to consider is the size of the tires. There's more to choosing the size than just matching them to your wheels. The size of the tires will affect your overall gearing, the power required and the weight of the wheel & tire combination.
As you get larger tires, you will need a more powerful engine to be able to get enough power to keep you moving. This is also true with more aggressive tread patterns, since they will dig into the mud more and require more power to drive them. Most tire manufacturers, such as Mud Trac, Super Swamper or Mudd Shark, will offer a minimum recommended engine size and/or horsepower rating for a given size of tire.
As you get larger tires, you might also want to consider some lightweight aftermarket wheels to help offset the additional weight.
The second thing to consider is the tread pattern. All mud tires are not created equal, and some have more aggressive treads than others. Typically as you get more aggressive treads, you'll need more power, you'll have a rougher ride and you'll also get a lower tire life. You might want to strike a balance between mudding power and tire life, otherwise you could find yourself replacing them far more often.
The third thing to remember when buying new tires is whether mud tires are really the best option. If you only ride in mud, they're ideal, but if you do any cross-country riding or even street driving, they're not the best choice. For one thing, they may not be DOT approved for use on the street so that could limit you. And the ride quality will suffer severely if you're riding on smoother surfaces with mud tires.