What is the difference between gravitational, electromagnetic, and nuclear forces?
The four, so called force, all operate at different ranges and on different particles in different ways.
First of all the term force is not really accurate for the four interactions. A force is something which causes objects to accelerate. Only one of the interactions actually does this and this is only a part of the possible interactions.
Electromagnetism is the interaction between charged particles. It is very long ranged. It can manifest as a force causing like charges to repel and unlike charges to attract. Electromagnetism also describes light and why atoms, which are mainly empty space appear solid. It is very strong.
Gravity is also very long ranged. It appears to be a force, but is actually a consequence of matter and energy curving the fabric of spacetime. It is weak but dominates at stellar and galactic distances.
The strong nuclear force is very short ranged. It is actually the residual effect of the colour force which binds quarks into hadrons and mesons. It is very short ranged and can only bind adjacent quarks.
The weak nuclear force is very short ranged and is not a force at all. It is responsible for radioactive beta decay. It can convert a proton into a neutron, a positron and an electron neutron neutrino. It can also convert a neutron into a proton, an electron and an electron antineutrino.