like you said aside from the obvious with that being that they are all types of covers on the balls. Pearls are typically a more polished/shiny ball with a reactive coverstock that will go further down the lane and snap towards the pocket when it sees the dry.
A solid is pretty much all of the dull balls that you see in the Pro Shop
; those balls typically hook by absorbing the oil from the lane and creating traction in the oil. They typically start hooking a little early than the pearls do.
A Urethane ball is an entry level ball that typically doesn't have much of a core in it. It has a cover on it that is a little softer than a plastic ball so it will still absorb oil, but not as much as a solid ball so the hook potential on the Urethane is a lot less than a solid or a pearl.
A reactive ball is basically the class of the above three covers. Or at least I think it is.
There are Shiny solids too (ever heard of polish?) Look at a brand new Brunswick Red Zone (Solid) and tell me it's dull.
In fact pearls tend to be shiny (but it's not because a ball is shiny that it's a pearl). They are reactive resins with tiny pearl like additives in the resin which, en general, make the ball go longer before it moves towards the pocket.
Urethane was the state of the art ball before reactive resin came out. They tend to hook less, generally speaking (but that's not always the case), but they CAN and mostly DO have a good core in them.
Generally speaking reactive resin is a solid, reactive pearl is (doh) a pearl and hybrid is a mixture of the two (1 part pearl and two parts solid or two parts pearl and one part solid)
There are also particle balls too with can be either solid or pearl (basically the same as their non particle counterparts but with small particles added) with helps them tackle more oil (like snow tires on a car).