In English, information is uncountable, so an information is incorrect, but I often see the French equivalent une information (as in donner une information).
Why is this correct? Should I prefer the equivalent of some information, e.g. quelques informations or des informations?
In French there is no notion as strong as the English uncountable nature of certain things. If you can think about a way to count something, you can speak about it as a countable.
For example, you can separate information in pieces of information, and it makes it countable: in French you say "une information" for "a piece of information".
Same goes for bread for example. Bread can be considered as an uncoutable and you will say "Je vais acheter du pain" if you are going to buy some bread but don't know exactly how much. However if you want someone to bring you exactly two loafs (actually bread in France is not in loaf shape, but let's keep it simple) you say "Apporte-moi deux pains".
One some case the countable and uncountable meanings are differents, like for the word "eau" (water). "de l'eau" means "some water", and no one would think about counting it, however there are expressions as "entre deux eaux" or "perdre les eaux" that refers to specific meanings.
Anne has a nice, elaborate description, but these type of classifications (count vs. noncount, but also genders, alienable vs. inalienable possession, regular vs. irregular, count words, deponency etc.) really come down, ultimately, to arbitrariness. Yes, there may be historical factors (although this just pushes the question to the parent languages), but there is really no all-encompassing explanation, and asking why one language goes one way and another goes another way usually is no more constructive than asking why tehre are different languages in the first place.
In French there are very few words that are uncountable. I can think of sand. You can't say "a sand / un sable". However you could say "a grain of sand / un grain de sable" or "a quicksand/ des sables mouvants". Note that in the last case the French still use plural.
