On a metric they are in milimeters, centimeters, decimeters. On an imperial they are in Inches, Half Inches and Quater inches on most of the scale, towards the lower end they go down to Eighth of an inch and Sixteenth of an Inch.
Different rulers have different measurements. 1 foot ruler has inches, centimeter, and millimeter lines. 1 millimeter is the smallest length in one foot ruler. On the other side, you will see lines of inches. There will be 12 lines there, and each line is 1 inch in length. It depends on the type of ruler you have.