There are a multitude of different reactor types but two very high level groups are solid fuel vs liquid fuel.

Solid fuel has been used in almost all if not all commercial reactors in the world. In these reactors the fuel assemblies are generally made of uranium and are a solid component. This means that during regular use of the reactor there is waste heat that needs to be removed. If this doesn’t happen the fuel melts creating an environmental catastrophe. Most solid fueled reactors also operate at high pressures. This makes safety systems for the reactor very complicated and require lots of redundancy.

Liquid fuelled reactors use an already liquid fuel. Waste heat still needs to be removed from the fuel but melt down is not possible. This also has passive safety features as you can create systems that can dump the fuel into tanks where criticality of the fuel stops and the heat can passively removed. The fuel is also at at atmospheric pressures with simplifies safety designs.

There have been a few research reactors proving the liquid fuel concept but there were technical challenges that needed to be overcome specifically with the corrosive nature of the high temperature salts they use as fuel.

Ultimately the industry went with solid fuel for a multitude of reasons. Technical challenges were only a portion as it’s not like todays reactors are simple. You also have to realize that todays reactors were chosen originally because they created plutonium for bombs. The liquid fueled thorium salt reactor was one of the first reactors tested but abandoned ultimately because it didn’t create plutonium well.