A pH of 7 is Neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic

Each whole pH value below 7 is ten times more acidic than the next higher value. 

For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10 times 10) more acidic than a pH of 6.

The term pH refers to the concentration of hydrogen ions (H+) in a solution. An acidic environment is enriched in hydrogen ions, whereas a basic environment is relatively depleted of hydrogen ions. The pH of biological systems is an important factor that determines which microorganism is able to survive and operate in the particular environment. While most microorganisms prefer pH's that approximate that of distilled water, some bacteria thrive in environments that are extremely acidic.
The hydrogen ion concentration can be determined empirically and expressed as the pH. The pH scale ranges from 0 to 14, with 1 being the most acidic and 14 being the most basic. The pH scale is a logarithmic scale. That is, each division is different from the adjacent divisions by a factor of ten. For example, a solution that has a pH of 5 is 10 times as acidic as a solution with a pH of 6.
The range of the 14-point pH scale is enormous. Distilled water has a pH of 7. A pH of 0 corresponds to 10 million more hydrogen ions per unit volume, and is the pH of battery acid. A pH of 14 corresponds to one ten-millionth as many hydrogen ions per unit volume, compared to distilled water, and is the pH of liquid drain cleaner.
Compounds that contribute hydrogen ions to a solution are called acids. For example, hydrochloric acid (HCl) is a strong acid. This means that the compounds dissociates easily in solution to produce the ions that comprise the compound (H+ and Cl). The hydrogen ion is also a proton. The more protons there are in a solution, the greater the acidity of the solution, and the lower the pH.