Acids are substances that break apart in water in a way that releases H+, also known as hydronium, ions. Hydronium ions are basically just a proton, and they will react with anything that they can pull an electron off of, for instance a metal. Since electrons are one of the basic particles that make up atoms, removing them from things can be a powerful reaction; we use this reaction in many processes, from helping to break down food in the stomach to storing electricity in car batteries.
Bases are substances that release OH-, or hydroxide, ions in water. These are able, because of their negative charge, to disrupt complex molecules like proteins. Although most people realize that acids can be dangerous to handle, sometimes they don't realize that a base can be just as dangerous. We have bases in our saliva to protect our teeth from acidic foods, and we use them as drain cleaners and paint strippers.
The pH scale (think "power of Hydrogen") tells us mathematically how much H+ is present in a solution. The scale runs from 0 to 14. 7 is neutral, any solution with a pH below 7 is an acid, and anything above 7 is a base. Because the scale is logarithmic, it is based on powers of 10. This means that a solution with a pH of 5 will be 10 times more acid than one with a pH of 6. A pH of 4 is 10x10, or 100, times more acid than pH 6.
If you need to learn to do the calculations for the pH of solutions the link below will help you to understand it.
The pH scale is used to measure how acidic or basic a substance is. The pH scale focuses on the concentration of H+ (hydrogen) ions. Remember an ion has lost or gained electrons. The pH scale goes from 0 to 14. Distilled water has a pH of 7, which is neutral.
Acids are from 0 to 7 and bases are from 7 to 14. An acid is a colution that has a lot of H+ ions. It comes from the a latin word that means sharp/sour.