Now I'm curious, what do they mean in America? As a Canadian I think both words describe a single unit in a multi-story building, but say "apartment" if the entire building is owned by one company and all units are rented, and "condo" if each unit is sold to an individual owner (who may still rent it out privately if they want). I rent an apartment, I'd buy a condo. They're physically the same thing, just different ownership models. Most Canadians I know (Vancouver/Calgary) do the same.
What I have encountered is that the word apartment is accepted for any residential unit while words that specifically relate to the nature of occupancy are not always used because that may be ambiguous. Is an apartment the only residential unit in the building? Is an apartment a condo, joint or fractional ownership, rental, or timeshare? All of these tend to be referred to casually as apartments unless more is known and needs to be communicated.
It was really weird moving to the states and have people care which word I used.
(Extra) Fun Fact: Canadians do differentiate (a lot) between University and College. Something Americans do not do.