let's count the blatant lies and fearmongering
Croup and whooping cough are both infections that are common in children and it is a common misconception in most people that both are the same. Many people use both the terms interchangeably but it's worth noting that they are indeed different. The thing is that its not very easy for a lay man to tell the difference without an examination i.e. only by analyzing the symptoms. Before talking about the difference, it is better to know the symptoms of both.
Both coughs are characterized by very harsh sounding coughing, often accompanied by wheezing in the chest or noisy breath. Croup is due to a viral infection that results in the inflammation of the windpipe tissue. The narrowing of the windpipe results in a tight and barky sound to the cough. Since it is viral infection there isn't any proper vaccination against it. Whooping cough on the other hand is a bacterial infection caused by the Pertussis bacteria. Whooping is characterized by continuous coughing till the lungs are emptied and then inhalation with a 'whoop' sound or gasp. This is where the primary difference can be noticed between the two: the sound. Croup is hoarse seal-like bark whereas whooping cough has a high pitched gasping sound.
Plus most kids show mild symptoms of croup. Whooping is much worse, and very painful. Very severe croup can sound like that, but it's rare. Very young infants often don't do the whoop. Instead they just stop breathing and turn blue, which is dangerous. Another thing that is common in whooping cough is that bouts of cough may end in vomits, which doesn't happen in croup.
The good thing is that there is a vaccination available for Pertussis - the DTAP vaccine - which drastically reduces the chances of getting the infection. So if your little one got the DTAP shot, the probability of whooping is very low. But if you haven't got him the DTAP, you need to be worried. In any case, if there is fever, you definitely need to go see a doctor. Croup is not as dangerous as whooping, which can and has killed many little innocents. Please if you have a doubt, consult a doctor.