pH: ACIDS VERSUS BASES

Measure of Hydrogen Ions (H+) in solution: The more Hydrogen Ions (H+) there are the more acidic a solution is.

Water

Where do Hydrogen Ions (H+) come from? Well, it's all part of water really...

• Water (H2O) splits into Hydrogen Ions (H+) and Hydroxyl Ions (OH-).
• When there are equal parts of Hydrogen Ions (H+) and Hydroxyl Ions (OH-) leading to a 1:1 ratio, pH is neutral (7).

Sometimes other chemicals are present that are dissolved by water. The pieces that result may contain an Hydrogen Ion (H+) or Hydroxyl Ion (OH-). This will change the pH.
Acids
Acids add Hydrogen Ions (H+) to solutions.

• Hydrochloric acid (HCl) splits into Hydrogen Ions (H+) and Chloride Ions (Cl-). 
• Extra H+ means acid solution  (no more equal parts).
• the 1:1 ratio is changed, now there are too many H+, it turns acidic.

Bases
Bases add Hydroxyl Ion (OH-) to solutions.

• Sodium Hydroxide Solution (NaOH) splits into Sodium (Na+) and Hydroxyl Ions (OH-).
• Extra Hydroxyl Ions (OH-) shifts ratio (fewer free H+ than normal).
• the 1:1 ratio is changed, now there are too few Hydrogen (H+) and there are "extra" OH- ions.
• The solution becomes basic.

Anything that's too acidic or too basic will degrade organic matter. Tissues are destroyed, cells will die or at least not function properly.