When added to water, how does an acid affect the pH and H+ concentrations? What increases or decreases and why?

pH is a measure of acidity or basisity (alkalinity) of a
solution.  It is approximately equivalent to the negative log of the concentration of
hydronium (H3O+) ions. Because of the negative log relationship, increased hydronium
ions translates to decreased pH.

When added to water, an
acid releases hydrogen ions.  Hydrogen ions in water combine with the water molecules to
form hydronium ions, thereby decreasing the pH and increasing the acidity of
water.

Water exposed to air is mildly acidic because it
absorbs small quantities of CO2 from the atmosphere.  The CO2 breaks down to carbonic
acid that then dissociates to release hydrogen ions into the water, decreasing the
pH.

To answer your question, acid added to water increases
the hydrogen ion concentration (H+), decreases the pH, and increases the
acidity.