mr.me, I want to appreciate you for putting this question up in the sense that the question is very simple but baffling to comprehend. Most of the us are confuse about these words. What is the exact meaning of these words and how could we discriminate them? These words are related to each other but there is a divergence between these words and their insinuation. Without understanding the meaning of these words, how can we include symptoms of the patient and how are we treating the patients?
Cold is a virus infection of the URT (upper respiratory tract), with inflammation in the nose, throat, larynx, and often the trachea and bronchi. This is a disease. In typical pakistani language we call it "SARDI LAGNA"
Catarrh is an old word shows or represents the condition / stage of the disease. When the mucous lining of URT will get inflammed then this condition is called catarrh. This will come in the first stage of infection. This is not a disease but refers to a condition of copious body cleansing, such as a runny nose, where the body exercises natural methods to rid itself of germs or a toxic condition in the sinuses. It is the action of the runny nose. In Pakistani Urdu language this is called "ZUKAAM". Just like sneezing, this also refers to reflexive reaction of a disease and not a disease itself. Fever is not a disease but a symptom of a disease. You can say Catarrh is a symptom and not disease. Catarrh is thinner and not offensive accompanied with sneezing and coughing.
Coryza This also a type of condition of a disease but comes in the secondary stage of infection. This generally caused by catching cold. It is characterized by an increased yellow dark discharge from the nostrils. Coryza is thick and offensive accompanied with blockage of nose etc.
Influenza is a severe cold caused by virus. This is also separate disease. You can find lot of information on net about this disease.
I hope you will continue your studies.