What’s the difference between catarrh, mucus and phlegm?

Mucus is the slippery fluid that lines your nose, throat and sinuses to help trap dust, allergens and pollution. Catarrh is a symptom caused by the build-up of mucus. Phlegm is a thicker type of mucus coughed up from lower in the lungs, although sometimes you do hear the term used interchangeably with mucus.