Algebraically, the following are equivalent expressions of Ohm's law:

While each of these are equivalent, they represent different ways of thinking about inputs and outputs. We usually think of the variable that is isolated (and usually put on the left) as the output. The stuff on the right side of the equals sign includes the input, but it often also includes constants (or coefficients).
Let's look a little more closely at the first equation. Even if we take voltage to be the output, we have another choice. We can take current to be the input while resistance represents the constant of proportionality between voltage and current, or we can take resistance to be the input while current is the constant of proportionality between voltage and resistance.
The same choice is available for each of the three versions of Ohm's law. That means there are six different ways to think about and represent Ohm's law!
For now, let's just look at three of these.