pH (Potential of Hydrogen) is a scale of acidity from 0 to 14. It tells how acidic or alkaline a substance is. The lower the pH, the more acidic is the solution. The higher the pH, the more alkaline is the solution. Substances which are not acidic or alkaline (neutral) usually have a pH of 7. Acids have a pH less than 7. Alkalis have a pH greater than 7.
There are a lot of ways for finding the pH of some chemical. One way is by using a red or blue litmus paper. The pH paper is able to tell how strong the chemical is, whether it is a stronger acid or a stronger base.
In fact, pH is a measure of the concentration of protons (H+) in a solution. S.P.L. Sørensen introduced this concept in 1909. The p stands for the German potenz, meaning power or concentration, and the H for the hydrogen ion (H+).
[H+] indicates the concentration of H+ ions (also written [H3O+], concentration of the equivalent hydronium ions), measured in moles per litre (also known as molarity).
Most substances have a pH in the range 0 to 14, although extremely acidic or basic substances may have pH < 0, or pH > 14.
Basic substances have, instead of Hydrogen ions, a concentration of Hydroxide ions (OH-).
Some common pH values
Battery acid 1.0
Lemon juice 2.4
Orange or apple juice 3.5
Acid rain < 5.6
Pure water 7.0
Blood 7.34 - 7.45
Sea water 8.0
Hand soap 9.0 - 10.0
Household ammonia 11.5
Household lye 13.5
For More Information: pH K-12 Experiments & Background Information
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)