1.
A healthy mouth has firm, pink gums that don't bleed when probed, flossed or brushed.  The gum is not attached to the tooth right from the top.  There is a 1-3mm deep crevice around the gum line called the gingival sulcus.  Flossing correctly cleans this area.  Your toothbrush will not reach into the sulcus, so it is important to floss and brush daily.

2.
When plaque bacteria accumulate in the sulcus the gingival crest becomes red and inflamed.  It bleeds easily when probed, flossed or brushed.  If the gum attachment starts to give way the probe will penetrate deeper into the sulcus.  This is now a pathologic condition known as pocket formation.  This can be treated with scaling and polishing and improving oral hygiene.

3.
As plaque accumulates it begins to calcify from the minerals in your saliva.  This rough, hard deposit is called tartar.  The mechanical damage tartar causes the gums along with increasing amounts of bacterial toxins causes the bone supporting the tooth to be eroded away.  The gums begins to recede and the tooth loosens.  To treat this deep scaling, root planing and polishing accompanied by improved oral hygiene is needed.

4.
An advanced stage of periodontitis can be reached without much pain despite what the mouth may look like.  Patients with this condition may not even be aware of the bad breath that accompanies the chronic puss drainage.  The teeth are so mobile and the bone damage so advanced that the teeth will probably need to be removed to clear the infection.