Really, there's plastic, urethane (not many), and reactive. Reactives, when you get down to it, are just urethanes with a reactive additive. Particles balls are just reactives with a particle additive. The particles might be rubber, silica, glass, even industrial grade diamond.
Pearls are just reactives with a pearl additive (mica).
In most cases, you can visually tell the difference between a pearl and a solid. Pearls are kinda sparkly, they have an iridescent sheen to them. Solids are flat. That doesn't hold all the time, though, and some of the newest colors look pearlized but aren't. The Big-R-Bang and World Class Reactive have a definite pearl look but are supposedly solids. I had a Black Sparkle Power Groove and it looked, well, sparkly, but I believe it was a solid.