There are lots of differences. Here are some of the main ones:
Mountain bikes have fatter tires, which are not as highly inflated, to give you better traction on dirt.
They often have a lower granny gear, for more power going up hills.
They commonly have shock absorbers for one or both tires.
They’re sturdier, less delicate.
Handlebars are usually horizontal, not dropped, as they are on some road bikes.
Mountain bikes are usually smaller, for a person of a given height, than road bikes.