Blepharitis is an inflammation of the eyelids in which they become red, irritated and itchy and dandruff-like scales form on the eyelashes.  It is a common eye disorder caused by either bacteria, a skin condition (such as dandruff of the scalp or rosacea) or demodex (eyelashes mites).  It can affect people of all ages.  Although uncomfortable, blepharitis is usually not contagious and generally does not cause any permanent damage to eyesight.
Blepharitis is classified into two types:
Anterior blepharitis occurs at the outside front edge of the eyelid where the eyelashes attach.
Posterior blepharitis affects the inner edge of the eyelid that touches the eyeball.
It is currently unclear whether blepharitis causes dry eye or dry eye causes blepharitis.
Blepharitis is often associated with either or both:
Demodex mites:  when bacteria multiply and overgrow at the margins of the eyelids and at the base of the eyelashes, a biofilm is formed.  Biofilm is a toxic environment similar to plaque that forms on your teeth, from which parasitic eyelash mites called demodex can feed on.  When untreated, demodex create a vicious cycle that worsens the eyelid inflammation and causes more dry eye symptoms.
Meibomian gland dysfunction:  bacteria in the eyelid biofilm produce substances called exotoxins that cause inflammation of the meibomian glands. Over time, inflamed meibomian glands become obstructed, which causes (and worsens) dry eye discomfort.