Simply put, mucus is a noun and mucous is an adjective. So, the actual fluid that comes out of your nose when you are congested is mucus and the linings in your body that secrete mucus are mucous membranes.
Dictionary.com's definition of mucus is: ‘the slimy protective secretion of the mucous membranes, consisting mainly of mucin*.’ The word actually derives straight from the Latin word mucus, meaning ‘snot, slime, mold.’
Mucous, on the other hand, according to Dictionary.com, is ‘pertaining to, consisting of, or resembling mucus’ or ‘containing or secreting mucus.’
Coming across it in the above definition of mucus, I had to look up the word mucin. Mucin is ‘any of a class of glycoproteins found in saliva, gastric juice, etc., that form viscous solutions and act as lubricants or protectants on external and internal surfaces of the body’ (Dictionary.com).
There you have it, folks, probably more than you ever wanted to know about mucus-related words. For more about mucus itself, stay tuned for February's allergist interview, in which Dr. Scott Carroll of the Atlanta Allergy and Asthma Clinic discusses what those different colors of mucus mean for your health.