Yeah, there are different classes. "Open" class has no restrictions on airplane design, just engine displacement and airplane weight (less fuel). In B class, you can have up to a .30 size engine and 3.5lb max weight. There are also 1/2A, A, C, and SSC classes.

"Scale" class planes have to be 1/12th scale models of real airplanes. In AMA/RCCA rules, the plane has to have existed between 1935-1955, so it's ww2 with a large margin of error on both sides. In "2610", the only common scale sub-class, the models can be scaled up or down by up to 10%, and markings have to be close to belieiveable, but you don't have to model a specific airplane. In fact, you can make up squadron colors and such, and the rules are usually lightly enforced. Engines can be up to a .26

Scale does look cooler, but the planes are harder to fly, faster, and less tolerante of pilot error. They are also harder to build and not as durable as open class planes, so scale isn't as popular over all as open class. But it does have the highest "cool factor".

Here's a picture of my B class open fleet and my 2610 scale fleet at a contest earlier this year. Note the abuse and damage on the scale planes, they've been though the wringer. The front P-47 was knocked out shortly after this picture was taken.