Periodontal disease is a type of infection.  If you do not clean your teeth for a day you will find sticky, clear-white, material on the teeth called plaque.  If you look closely at this plaque you will see it is made of little lumps.  Each lump is actually a colony of bacteria that has grown from one tiny bacterial cell.   

Gingivitis (Infection of the gum): If not removed periodically,  this bacteria/plaque takes on a life of it's own.  It accumulates and changes in nature.  New and more virulent bacteria move in and it becomes more harmful to the tissues around the tooth.  It becomes an entrenched little "town" of bacteria, the bacteria all communicating with each other in a network. This is called a "biofilm."

When plague is allowed to flourish, it irritates the gums around the tooth and they respond to this infection by becoming inflamed and swollen with blood.  Thus,  they bleed easily. This has a cyclical effect: bleeding and soreness make brushing and flossing painful so the patient avoids cleaning well around and under the gum,  allowing the infection to just get worse.

Plaque in some mouths becomes calcified with minerals from saliva if it is allowed to remain on the tooth for a while.  This hard material is called calculus.  It complicates removal of the bacteria and must be professionally removed.

Gingivitis is an early form of gum disease that can be reversed with good cleaning of the tooth and proper plague removal at home by the patient.  Brushing, flossing, tooth picks, etc. are all meant to facilitate removal of plague.  Electric toothbrushes are an excellent aid to good home care, as is floss.  Regular cleanings at the dentist are needed to remove stain and calculus.

Periodontitis is the term used when the disease begins to affect the bone under the gum.   In response to a maturing biofilm/plaque under the gum tissue, and its bacteria that secrete irritating toxins,  the bone (in susceptible patients) begins to dissolve.   This beginning dissolution, or resorbtion, of bone can often be seen on an x-ray. Periodontitis can continue its destruction, resorbing more and more bone, until it becomes advanced.  In advanced stages of the disease, teeth become loose and gum abscesses may occur.

Not all people respond to the presence of plaque in the same way and this response is largely genetic.  Also, periodontal disease progresses at different rates in different mouths, sometimes being "active"  and sometimes less.  Some plaque contains more virulent and destructive bacteria and there may be a place for antibiotics in treatment. 

The drawings on the left illustrate the progression of gum / periodontal disease:

Drawing 1 shows  gingivitis.  Plaque accumulated on the teeth and under the gum irritates the gum which becomes inflamed and bleeds easily.  At this early stage, the disease is reversible with good home plaque removal by the patient and a professional cleaning.

Drawing 2 shows moderate periodontitis.  In response to plaque that has matured, or become more destructive, the gum becomes engorged with blood as the bone underneath dissolves.  This situation will require more than a cleaning and good home care. 

Drawing 3  shows advanced periodontitis.  The gums are swollen and the bone dissolution is much more severe.  At this point the teeth may be loose and gum abscesses might occur. 
Notice that the bone between the roots of the molar tooth is dissolved.  This can become a trap for debris and a non-cleanable area that is hard to treat.  It is called a "furcation" involvement.  Areas where the bone is missing are called periodontal pockets.  These pockets are increasingly hard to clean as they get deeper.