What is the difference between condominium and apartment?

[Terence Nash]

I have always thought that condos are a part of a building that you own and have a deed to while an apartment refers to an entire building where the owner(s) own the whole building.  

But every once in a while, I hear that someone is selling an apartment.  I assume he is selling an entire building.  Instead, he is referring to a single unit.  Are both words universal?