An ion is an atom or molecule that has an electrical charge. When an atom or molecule loses an electron, it gains a positive charge, and when a chemical gains an electron, it gains a negative charge. Ions are usually designated by their chemical name or formula followed by a + or - sign, and perhaps a charge number. An ion can loose or gain more than one electron. A chemical in it's ionic form may have different chemical properties from it's electrically neutral forms. The following discussion is geared to be as useful as possible for understanding and explaining physiological and environmental phenomenon.
One of the simplest ways to make an ionic solution is to allow some salt ( NaCl ) crystals to dissolve in water. In it's crystalline form, salt is neutral, as the two ions of the ionic crystal, sodium and chloride, cancel each other out. Once in solution, the sodium ( Na+ ) loses an electron and the chloride ion ( cl- ) gains an electron. A property of ions in aqueous ( water ) solution is that they are conductors of electricity. The conductivity a solution can be measured with special meters or with a bench top multimeter that measures resistance. A unit of conductivity is the inverse ( 1 / resistance ) of resistance and is therefore is in units of ( 1/ohm ).
Conductivity measurement is very common in the field of oceanography and estuarine ecology. If you were a oceanographer or a marine biologist collecting water samples for biological and chemical analysis on a transect across an estuary, the conductivity of each sample would be a good indication of the degree of mixing between ocean and fresh water sources.
Another pretty important chemical measurement related to ionic chemistry is pH. Like sodium and chloride in salt, water itself ( H20 ) can disassociate into two ions H+ ( a proton) and OH- ( hydroxyl ). In aqueous solution, these two ions are related by the formula the concentration of H+ times the concentration of (OH-) is equal to 10 to the negative fourteenth power in scientific notation. For example, of the concentration of H+ is 10 to the negative third power, we could calculate the the concentration of OH- is ten to the negative eleventh power. Ten the the negative third times ten to the negative eleventh power equals ten to the negative fourteenth power. We add exponents to get a solution.
This concept gives rise to the pH scale. The common scale runs from 1 to 14, and represents the negative exponent of the concentration of the hydrogen ion, or proton. In the example above, a solution with a H+ concentration of ten to the negative third power has a pH of 3.
Ions in solution can interact with each other. This is an important concept, and in fact fundamental to the field of physiology. In all organisms, correct biochemical and electo-physiological function is determined by pH, among other factors. In order to help maintain a constant pH, biological solutions make use of a chemical principle called buffering. In physiological systems, the bicarbonate ion ( HCO3- ) is the most important. Bicarbonate is in chemical equilibrium with carbon dioxide ( CO2 ) in blood. Carbon dioxide combines with H20 giving rise to carbonic acid. Carbonic acid dissociation gives rise to bicarbonate and hydrogen according to the following formula:
CO2 + H2O <=> H2CO3 ( carbonic acid ) <=> H+ + HCO3- ( bicarbonate)
We notice that carbon dioxide ( CO2 ) at the left of this equilibrium can be thought of as the driving or controlling component of this equation, at least as it relates to respiration. We know that as we breathe, we exhale carbon dioxide, and inhale air rich on oxygen. As we remove carbon dioxide from our blood stream, we move the above equilibrium to the right, and reduce the acidity, or raise the pH of our blood. The phenomenon is actually familiar to us. When we exercise, we create lactic acid in our muscles and our blood stream which tends to acidify, or drive our blood's pH down. We compensate for this by hyperventilating, or breathing faster. This relationship although long presumed, now appears to be confirmed. 
Individual cells must transport ions to maintain their polarity. A cell maintains a slight voltage across it's membrane, and this is referred to as polarization. At rest, it is about -70 millivolts. The primary engine that maintains this polarization potential is a molecular pump in a cells membrane called an NaK ATPase. The name is fairly simple to decipher. Na is sodium, K is potassium, ATP in the energetic molecule that drives our biological processes, and ase means that this is an enzyme that uses ATP to pump Na and K across the cell's membrane. The NaK ATPase pump moves three Na+ ions out of the cell for every two K+ cells that are transported into the cell's cytoplasm. As such, at rest, a cell's cytoplasm contains little sodium and much potassium. Some cells, such as neurons and muscle cells can depolarize as a method of transmitting signals. Depolarization occurs when a neurotransmitter signals a cell to allow sodium ions to rush into the cytoplasm. The ionic and electrical gradient caused sodium to move across the cell's membrane, when it is free to, by open sodium channels, and when sodium channels open, the electrical polarization of the cell is reversed. Thus the cell manages electrical potential by managing IONS, and as a result, we can generate electrical signals in our body. Another name for the signal created by cell depolarization, and rapid movement of ions is action potential.
There is a special case in our body where all of the cells generate action potentials in synchrony. It is the beating of our heart. Because electrical signals are additive, we can observe the action potentials of our heart muscle by use of a system of electrodes and measuring devices called an electrocardiogram, or ECG, EKG.
There is a special caseThe study of electrical signals generated by a neuron's management of ions is referred to as electorphysiology, or neurophysiology.
I hope this has provided a guide to some of the key concepts of ions as they relate to physiological processes, and while not complete, I hope that it inspires you to put some effort into your understanding of ions because they are fundamental to physiology, which in turn is a fundamental science for all health care fields as well as biology, and even ecology.
 T Meyer, O Faude, J Scharhag, A Urhausen, and W Kindermann
Is lactic acidosis a cause of exercise induced hyperventilation at the respiratory compensation point?
Br J Sports Med. 2004 October; 38(5): 622625. [here]