Materials like copper, silver, and gold have very low resistances. In fact all metals will conduct electricity. They are called conductors. Even so, they do have some resistance to the flow of electrons through them. A perfect conductor is called a superconductor, it has zero resistance.
Resistance is measured in Ohms.
So if you know two of the values you can easily work out the third one. If you know the current and resistance and want to calculate the voltage, you use the first equation. If you know the voltage and resistance and want to calculate the current, you use the second equation. Lastly, if you know the voltage and current and want to calculate the resistance, you use the third equation. I have written a page about power which is measured in watts.