Mountain Biking - Is the difference in the wheels?

I'm stuck between a mountain bike and a hybrid. Obviously a mountain bike has superior shocks and a stronger frame compared to a hybrid bike, but other than that, is the difference between an entry level mountain bike and an entry level hybrid just in the wheels?

Is it safe if I were to get a hybrid bike for biking to school and around the neighborhood, and to change the wheels out to mountain bike wheels for going down light trails? Obviously I shouldn't be going around jumping giant rocks on a hybrid, but just going down gravel trails on hiking trails isn't bad, is it?

Hybrid bikes usually have 700 wheels for the application while mtbs are 26 inch. The space in between the seat stays are often not wide enough to fit tires that are as wide as mtb tires if were to think of getting knobby 700 tires.

So basically, mtb tires are too wide to fit in a hybrid?

Pretty much.

The wheel of the hybrid is taller also. Like a road bike.

I can't claim I've ever ridden a hybrid, but I think besides what's already been mentioned, hybrids also tend to put the rider in a more upright riding position.

It all depends upon the specific model of hybrid. Some do come in 26-inch wheels. Some are more towards the mountain end of the spectrum than others.

Differences: gearing on a hybrid is sometimes taller. For example, the last two hybrids that I bought ran 26/36/48 up front rather than 22/32/44. The geometry sometimes will have you more upright. The frame may not be quite as tough as on a mountain-bike (though I wouldn't stress too much on that point). One hybrid that I bought came with shorter crank-arms than comparable mountain-bikes; I'm convinced that was to encourage spinning on the road rather than mashing on the trail. Various parts such as tires may be oriented more towards pavement than dirt.

Some hybrids come without front suspension. And if there is a suspension fork, the travel may be less than on a mountain-bike. The fork may not be as rugged either.

If you want a trail-worthy hybrid, look at something like the Kona Dew or the Specialized Crosstrail. Either of those bikes would be fine for light trail riding.

Why not go the opposite route and get a mountain bike but put slicks on it for biking to school? Then when you want to do some trail riding, you know your knobbies will fit.

Same for the hybrid, you can get mountain tires in the 700c size (that's what 29ers are) and alternate tires for trail/street. Hybrid likely won't fit as wide a tire as a mountain bike, though. Depends on what your gravel trails are about I suppose, but I'd think a hybrid for street and gravel walking paths would be quite fine...

What would the difference between riding a mtb with slicks and a hybrid be? Say, up a hill?

Between entry level mtb and hybrids, is the weight difference negligible?

Comparing my old walmart Huffy from 3-4 years ago and a Trek 3700, the Trek is virtually half the weight. A hybrid is even lighter!

Now we are talking geometry (ease of hills) Hybrid geometry is waaay different from mtbs, depending upon usage. And since 700 wheels are larger, they cover more ground with each rotation.