pH

Usually, people will measure the concentration of  H⁺ in solution to determine the acidity, and measure the OH^- in solution to determine the basicity. But if people use the number like 1.0*10⁷, it is not simple enough to directly show the acidity and basicity. So, scientists create a way to help us understand the acidity and basicity concisely, that is pH scale.
pH=-log₁₀[H⁺]
For example, if a solution has 1.0*10⁷ M H⁺, then the pH=-log₁₀[1.0*10⁷]=7.

At the same time, we also have pOH.
pOH= -log₁₀[OH^-]
For example, if a solution has 1.0*10⁸ M OH^-, then the pOH=-log₁₀[1.0*10⁸]=8

However, generally, people only use pH instead of using both pH and pOH. The relationship between pH and pOH:  pH+pOH=14 (at room temperature 25。C)
That is because the product of concentration of H⁺ and OH^- always equal to 1.0*10^-14 M at 25。C.
[H⁺][OH^-]=1.0*10^-14 M 
So, we can know the pOH just by using 14 subtract the pH, and do not have to measure the concentration of OH^- in the solution.

Image from: Acids-and-Alkalis-The-pH-Scale.png