"They are buying trousers or a shirt."
Translation:Oni kupują spodnie lub koszulę.
immery's right, apart from one 'weird' exception: when we're not really sure about what we're talking about. If you were saying that "they went shopping, they're gonna buy some trousers, or a shirt, I don't know, and frankly I don't care", then you could use 'czy'. But I think this 'some trousers' should be used then. And translated to Polish.
"Oni kupują jakieś spodnie, czy koszulę, nie wiem."
Thanks for both of you! And Jellei, I think that the expression you gave as an example for when you can use "czy", is somehow linked to what Immery said, since it also represents the idea of "uncertainty". Both in questions and in your example, there is a sense of uncertainty. Is it correct to sum up the issue saying that you can use "czy" in every sentence that expresses uncertainty?
I think that may be too much, it's safer to stick to using it only in questions - my example is kinda unusual (although mentioned on English wiktionary).
There are two possible answers here:
In logic, in mathematics, I believe also in law: if you say "X lub Y" than either X is true, Y is true, or both are true. "albo" gives you just a choice between X and Y.
In everyday life, this difference is not really observed and they are used rather synonymously.