Well, very briefly, olfaction is the scientific word for the ability to smell. Odours and aromas are both things that you can smell but we use aromas for rather nice smells (coffee, herbs etc) and odours for unpleasant smells usually (like sweat, urine). Scent is perfume, or more generally a smell someone or something gives off (dogs follow the scent of a rabbit). Collocation is everything (collocation means words that are associated together often). In a scientific report expect olfaction. If the toilet is blocked expect odours. Opening a new coffee jar gives you a lovely aroma. Sweaty socks, as well as giving off a bad odour, will leave a strong scent for a dog to follow. You'll have to spray your socks with scent (perfume) !

In English, scent and aroma (particularly aroma) have a positive meaning, so you can think of them as "smelling good." Odor has a bad connotation, so you say something has an odor if it smells bad. Smell itself is neutral, but generally tends to be more negative than positive (when you say something smells, it means it smells bad. To make it positive, you have to say "it smells good.") Olfaction is a scientific word that is neutral and not used often in everyday life.