What is a "road" bike? Don't they all ride on a road? Trying to learn about bicycles.  I have heard the term "road" bike a lot.  What is the difference between a "road" bike and a regular bike?
Yes, all bikes could ride on roads. However, a road bike is one that's optimized for riding on smooth pavement. It usually has skinny tires (no wider than 32mm, often much narrower) with a very light tread. (If they have any tread at all. Some road tires, such as the Kojac, are treadless or "bald", to cut down on rolling resistance.) Road bikes usually have handlebars that are designed to be comfortable for long stretches of time, sacrificing a little bit of fine control that you'd get with a wider flat bar or riser bar. 

By way of contrast, mountain bikes come with wider tires that have more agressive tread ("knobby" tires) designed to grip gravel or dirt. 

Road bikes can sometimes ride on packed dirt or gravel, but the ride will be very harsh. A bike optimized for off-road will be easier to control and can ride on surfaces that'd be problematic on a road bike, such as rocks, gravel, loose dirt, or pockets of sand. There are even bikes that will accept extra-wide tires (3 - 4"). People who ride on very bad roads that have had potholes and cracks installed copiously may prefer a mountain bike to a road bike, or may decide to get a hybrid bike, which is exactly what it sounds like. 

In the end, road bikes can sometimes do a little bit of off-roading, and mountain bikes can ride on the pavement, but these kind of bikes have different strengths.