An n-tuplet is a division of a given pulse in n even parts; hence, a quintuplet is playing 5 notes in the space of one, usually a quarter note, as a 13-tuplet is dividing a note in 13 even parts.
What I don't get, is that I have seen some tuplets put as 2/5 or 3/5 (often when dealing with polyrythms) — that is, playing tuplets of a certain number of figures, rather than unit divisions.
Could someone explain to me this, and how to feel and play them?