pH

 

Water molecules can dissociate into Hydrogen ions (H+) and Hydroxide ions (OH-).  Even pure water will have some of these, but in pretty much equal amounts.
Unequal amounts make a liquid acidic or basic/ alkaline.

Acids are substances that donate H+ ions to the solution:

Bases are substances that donate OH- ions to the solution:

The pH scale is a logarithmic scale.  So, for every "1" on the scale, it's really a power of 10.  A four is 10 times more acidic than a 5, 100 times more than 6, and 1,000 times more than 7.

Source

 Equations graphics used from: www.all-about-ph.com