Reactor grade material is usable in most nuclear power plants.

Weapons grade material is required for nuclear weapons.

For uranium the difference between reactor grade and weapons grade is the level of enrichment: less than 20% uranium-235 is reactor grade, greater than 20% uranium-235 (greater than 90% is prefered) is weapons grade.

For plutonium the difference between reactor grade and weapons grade is the level of contamination with plutonium-241: any amount of plutonium-241 is OK for reactor grade, only low levels of plutonium-241 are acceptable in weapons grade as its spontaneous fission rate can cause the bomb to fizzle.

Uranium and plutonium can be used as nuclear fuels for nuclear reactors.

This part is the core of the nuclear reactor containing the nuclear fuel.

neptunium and plutonium

Yes, the nuclear reactor can be useful when it comes to making nuclear weapons. Uranium can be lowered into the operating reactor and can be bathed in the neutron flux to become (through nuclear transformation) plutonium. Plutonium is ready to be shaped into the subcritical masses used in nuclear weapons.