Phlegm is the mucus secreted by the glands in the respiratory passages and the lungs. Phlegm sometimes exits through the mouth when you cough, but is not the same as the mucus produced and excreted from the nasal passages(rhinitis or post nasal drip).
Phlegm can be thick or thin, sticky or watery, and may vary in color depending on infections, such as the common cold, allergies, level of hydration, and the climate you live in.