Cents: a logarithmic unit of measure used for musical intervals.
An equal tempered semitone is divided into 100 parts, resulting in an octave containing 1200 cents.

Ratio: a pair of positive integers, which is used to describe the relation between the frequencies of two sounds.
Those numbers can be applied to any frequency through multiplication to create certain musical intervals.
For example: 2:1 gives an octave, 3:2 is a perfect fifth and 4:3 is a perfect fourth.

Formula: cent = log_{2} ( frequency 1 / frequency 2 )