Mom This is the exception on the net, and Dean deserves a great deal of credit for that. Here its the basher that gets bashed, not the rookie.
Not there are a number of difference between the purpose of the bikes. Mountain bikes are designed for trails and the rough ground. To make matters worse the combinations of mountain bikes are pretty varied. Most (but not all) have 26 inch wheels and can have a fairly wide range of tires, with the general rule of thumb is that a wider tire works better on soft ground, and narrow tires on hard pack or pavement. The geometry of the frame is set up so the rider has maximum control over the bumps. The gearing is often set up for climbing.
The hybrid is a different beast all together. It's mostly a road bike. It usually has road bike gearing. It is usually faster than a mountain bike. It mostly has road bike type of wheels, but its not really a road bike. The geometry of the frame is set up to have more of an upright and comfortable riding position. The road bike will be far more aerodynamic but that comes at the price of comfort. The hybrid can handle improved trails, (those that are paved or really hard packed dirt) but those narrow tires are hardly what you need if you are riding out in the woods. For many walking trails it will be fine, but if you like to follow game trails to see where the deer have been, the hybrid isn't the right ride, but on the road it will be faster than a mountain bike and more comfortable than a road bike.