Gauge is the distance between the tracks. Z,N,Ho,O,G,F, are scales, or the ratio of size compared to the real thing. 1:1 is the real thing. Ho is 1:87, or 1/87th as big as the real thing. O is 1:43, or 1/43rd the size of the real thing.
If the scale has an (n) after it, that means it's narrow...