Does a hybrid truely have a wider tire or does a mountain have a wider tire? I'm looking around my local bike shop and saw that the hybrids have a slim tire while a mountain has a more fatter tire. And when i researched it says the opposite. Please help me.
What you saw is generally correct. In order of increasing tire size by bike type, it roughly goes:


road bike
hybrid
mountain bike


Of course there are exceptions and variations on these bikes that fit in that continuum, but as a general rule for tire width it's pretty accurate.